feat: add ESM export

[stipsan]

Adds module and exports to package.json.
The files referenced are generated using esbuild.

What this solves

A lot of new, modern, environments require packages to start shipping ESM.
For example when attempting to use @sanity/client in a Remix site will yield an error like the one below:

This is from same-origin, which is included by get-it, which is used by @sanity/client.
To solve this there's a new build command npm run esbuild:browser that transforms CJS to ESM and includes them in the bundle. This solves the problem, for now, buying us time to migrate these packages to ESM:

• https://www.npmjs.com/package/@sanity/eventsource
• https://www.npmjs.com/package/@sanity/generate-help-url
• https://www.npmjs.com/package/get-it
• https://www.npmjs.com/package/make-error
• https://www.npmjs.com/package/rxjs

Once they are shipping ESM we can stop prebundling them:

    "build": "npm run compile && npm run browserify && npm run minify && npm run esbuild:browser && npm run esbuild:node",
-   "esbuild": "esbuild src/sanityClient.js --bundle --sourcemap",
+   "esbuild": "esbuild src/sanityClient.js --bundle --sourcemap --external:rxjs --external:@sanity/eventsource --external:get-it --external:make-error --external:@sanity/generate-help-url",
    "esbuild:browser": "npm run esbuild -- --format=esm --outfile=dist/sanityClient.browser.mjs --platform=browser",
-   "esbuild:node": "npm run esbuild -- --format=cjs --outfile=dist/sanityClient.node.cjs --platform=node --external:object-assign --external:rxjs --external:@sanity/eventsource --external:get-it --external:make-error --external:@sanity/generate-help-url",
+   "esbuild:node": "npm run esbuild -- --format=cjs --outfile=dist/sanityClient.node.cjs --platform=node",
    "lint": "eslint .",

How to test

A test version is published under npm i @sanity/client@esm.

Using unpkg.com

Run this in your console (not on github as it blocks external scripts, but for example on https://sanity.io or a localhost):

const {default: createClient} = await import('https://unpkg.com/@sanity/client@esm?module=true')
createClient() // Should throw as no projectId is given

Using skypack.dev

const {default: createClient} = await import('https://cdn.skypack.dev/@sanity/client@esm')
createClient() // Should throw as no projectId is given

./dist/sanityClient.browser.mjs

This file is generated to run in ESM strict envs, like deno, native browser ESM using CDNs like unpkg or skypack. To do this it bundles everything (no import or require calls left) and it applies our pkg.browser field making sure to load browserMiddleware.js instead of nodeMiddleware.js.

./dist/sanityClient.node.cjs

Generated to run in allows calling require, using .cjs to make it clear to modern 'node' that it's a CommonJS file.

Once we have a bundler that implements module.createRequire we could consider adding a ./dist/sanityClient.node.mjs version.

pkg.module

This only affects bundlers that are modern enough to understand what it does, and does not trigger modern node to run in ESM module (unlike pkg.type: "module"). It points to dist/sanityClient.browser.mjs instead of dist/sanityClient.node.js as it has the highest chance of bundling success since pkg.browser is already applied to it.

adding package-check to posttest

Runs @skypack/package-check to validate we're following ESM best practices.

[stipsan]

Fixes a parse error due to the dynamic import() in client.test.js

[stipsan]

node v12 is dead, longe live node v12 👑