feat(metro-config): add postcss

[EvanBacon]

Why

• Required for modifying CSS and supporting Tailwind on web (but also universally).
• Eventually we'll want to add some postcss transforms by default (standard in the web community).

How

• Write a postcss parser which loads the user config.
  • postcss.config.js and postcss.config.json are the only supported config files. https://github.com/webpack-contrib/postcss-loader/blob/ef332db57381ed1d8aaa4a9a20eec0e0b21fdb7c/src/utils.js#L64-L114
• Process input CSS with postcss config and pass output to standard CSS loading process.
• TBD: More advanced Metro cache invalidation.
  • We currently have a basic hashing system which auto invalidates the transformer when the config file and browserslist options change.

Test Plan

• Added minor tests for certain parts of the flow but nothing e2e.

Checklist

• Documentation is up to date to reflect these changes (eg: https://docs.expo.dev and README.md).
• Conforms with the Documentation Writing Style Guide
• This diff will work correctly for expo prebuild & EAS Build (eg: updated a module plugin).

[evelant]

@EvanBacon This appears to be crashing builds when using EAS in a pnpm monorepo. I think this is relying on a phantom undeclared dependency on metro-cache so it crashes when it can't find the undeclared dep.

edit: on expo 49.0.0-beta.0

[RUN_GRADLEW] > Task :app:createBundleReleaseJsAndAssets FAILED
[RUN_GRADLEW] Error: Cannot find module 'metro-cache'
[RUN_GRADLEW] Require stack:
[RUN_GRADLEW] - /Users/imagio/Library/pnpm/global/5/.pnpm/@expo+metro-config@0.10.3/node_modules/@expo/metro-config/build/transform-worker/postcss.js
[RUN_GRADLEW] - /Users/imagio/Library/pnpm/global/5/.pnpm/@expo+metro-config@0.10.3/node_modules/@expo/metro-config/build/ExpoMetroConfig.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/start/server/metro/resolveFromProject.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/utils/analytics/getMetroProperties.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/start/server/metro/instantiateMetro.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/export/embed/exportEmbedAsync.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/export/embed/index.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/bin/cli
[RUN_GRADLEW] Error: Cannot find module 'metro-cache'
[RUN_GRADLEW] Require stack:
[RUN_GRADLEW] - /Users/imagio/Library/pnpm/global/5/.pnpm/@expo+metro-config@0.10.3/node_modules/@expo/metro-config/build/transform-worker/postcss.js
[RUN_GRADLEW] - /Users/imagio/Library/pnpm/global/5/.pnpm/@expo+metro-config@0.10.3/node_modules/@expo/metro-config/build/ExpoMetroConfig.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/start/server/metro/resolveFromProject.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/utils/analytics/getMetroProperties.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/start/server/metro/instantiateMetro.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/export/embed/exportEmbedAsync.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/export/embed/index.js
[RUN_GRADLEW] - /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/bin/cli
[RUN_GRADLEW]     at Module._resolveFilename (node:internal/modules/cjs/loader:1075:15)
[RUN_GRADLEW]     at Module._load (node:internal/modules/cjs/loader:920:27)
[RUN_GRADLEW]     at Module.require (node:internal/modules/cjs/loader:1141:19)
[RUN_GRADLEW]     at require (node:internal/modules/cjs/helpers:110:18)
[RUN_GRADLEW]     at getPostcssConfigHash (/Users/imagio/Library/pnpm/global/5/.pnpm/@expo+metro-config@0.10.3/node_modules/@expo/metro-config/build/transform-worker/postcss.js:242:7)
[RUN_GRADLEW]     at getDefaultConfig (/Users/imagio/Library/pnpm/global/5/.pnpm/@expo+metro-config@0.10.3/node_modules/@expo/metro-config/build/ExpoMetroConfig.js:281:56)
[RUN_GRADLEW]     at Object.loadAsync (/Users/imagio/Library/pnpm/global/5/.pnpm/@expo+metro-config@0.10.3/node_modules/@expo/metro-config/build/ExpoMetroConfig.js:304:23)
[RUN_GRADLEW]     at Object.loadMetroConfigAsync (/private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/start/server/metro/instantiateMetro.js:47:40)
[RUN_GRADLEW]     at exportEmbedAsync (/private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/export/embed/exportEmbedAsync.js:12:54)
[RUN_GRADLEW]     at /private/var/folders/gx/fwg565ys3177ht4tnx40wqv80000gn/T/eas-build-local-nodejs/13cda27c-eab6-4f56-9ee8-025b975f0361/build/node_modules/.pnpm/@expo+cli@0.10.4_expo-modules-autolinking@1.5.0/node_modules/@expo/cli/build/src/export/embed/index.js:112:16