feat: svelte config util

text/0000-svelte-config-util.md

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+- Start Date: (fill me in with today's date, 2021-09-28)
+- RFC PR: (leave this empty)
+- Svelte Issue: (leave this empty)
+
+# Svelte config utility
+
+## Summary
+
+Introduce a new utility package `@sveltejs/config` to offer tools in the svelte ecosystem a common way
+to read and validate svelte config files and provide config features to users in a consistent manner.
+
+## Motivation
+
+Currently, tools implement config reading separately (or not at all),
+leading to extra maintenance and more work for new tools who need to read it.
+
+Reducing this overhead and offering a place to enforce consistency
+and implement new features is the main motivation of this RFC,
+resulting in a better dx for both users writing svelte config files
+and tool authors consuming them via a simple common api.
+
+### Features
+
+#### for tools
+* async `loadConfig` function
+* defined namespacing (top-level keys owned by a tool)
+* validation utilities
+
+#### for users
+* `defineConfig` helper for intellisense like in [vite config](https://vitejs.dev/config/#config-intellisense)
+* `async` config support like in [vite config](https://vitejs.dev/config/#async-config)
+* allow `svelte.config.ts`
+
+
+## Detailed design
+
+### Technical Background
+
+### Prior Art
+* [postcss-load-config](https://github.com/postcss/postcss-load-config)
+* [sveltekit](https://github.com/sveltejs/kit/tree/master/packages/kit/src/core/config)
+* [vite-plugin-svelte](https://github.com/sveltejs/vite-plugin-svelte/blob/main/packages/vite-plugin-svelte/src/utils/load-svelte-config.ts)
+
+### Implementation
+
+The implementation is going to be fully compatible to existing usage of svelte.config.js (cjs/mjs) files.
+
+Tools can opt in by adopting `@sveltejs/config`
+
+New config features for users could potentially introduce changes that are not backwards compatible
+to tools reading svelte config without `@sveltejs/config`.
+To reduce friction, v1.0 of @sveltejs/config could be limited to features that are explicitly backwards compatible
+and features that would break will only be released after our tools adopted it
+
+#### New Features for tool authors
+
+##### async `loadConfig` function
+```js
+import { loadConfig } from '@sveltejs/config';
+//...
+const config = await load_config();
+```
+
+loadConfig accepts an optional `options` argument with the following options:
+```js
+options = {
+    fromDir: '/some/path', // process.cwd by default, tools can pass what they need, eg vite root
+    inlineConfig: {/*...*/}, // svelte config object built from inline options eg cli args passed to the tool. merged in
+    namespace: 'kit', // namespace for the tool loading the config, useful for advanced features
+    //...
+}
+```
+
+##### defined namespacing (top-level keys owned by a tool)
+
+Currently, namespaces in svelte.config.js are loosely agreed upon. 
+`compilerOptions`, `preprocess`, `extensions` and `kit` are used by SvelteKit.
+
+Each tool will get its own namespace, e.g. `vitePlugin`, `rollupPlugin`, `webpackLoader`, `languageTools`.
+
+Community provided tools will be allowed to use their own namespaces (e.g. `elder` or `routify`),
+but these are not supported beyond being tolerated by `@sveltejs/config`
+
+##### validation utilities
+
+SvelteKit contains [code](https://github.com/sveltejs/kit/blob/master/packages/kit/src/core/config/options.js) to validate config options and output helpful messages. 
+Refactor this code to be of general use and provide an api
+
+```js
+import { loadConfig ,validateConfig } from '@sveltejs/config';
+//...
+const config = await load_config();
+const rules = {
+    //TBD, use sveltekit options as a base
+}
+// throws on error
+validateConfig(config, rules);
+```
+
+#### New Features for users
+
+##### `defineConfig` helper for intellisense like in [vite config](https://vitejs.dev/config/#config-intellisense)
+```js
+import { defineConfig } from '@sveltejs/config'
+
+export default defineConfig({
+// ...
+})
+```
+
+##### `async` config support like in [vite config](https://vitejs.dev/config/#async-config)
+```js
+export default defineConfig(async () => {
+    const data = await asyncFunction()
+    return {
+        // build specific config
+    }
+})
+```
+
+##### allow `svelte.config.ts` and transpile automatically
+
+Transpilation is done via lazy dynamic import of `typescript` or `esbuild`. If neither is available, this fails.
+It allows users of typescript projects to fully embrace ts without burdening js projects with a typescript dependency.
+
+
+## How we teach this
+
+### To tool authors
+
+README.md and/or docs/ in the new GitHub repo, example usage in kit and vite-plugin-svelte
+
+### To users
+
+Currently, svelte.config.js isn't really documented on its own. The tools show how they use it.
+Add user documentation to the GitHub repo too and link to it from svelte.dev docs.
+Alternatively: Add a new section to the svelte.dev docs directly, this would incur extra work for new features though
+
+
+## Drawbacks
+
+Once a tool adopts this, it is bound to the features offered. 
+If new requirements come up it would have to be added in `@sveltejs/config`
+and create additional overhead for coordinating releases
+
+## Alternatives
+
+### use existing 3rd party tools
+[lilconfig](https://github.com/antonk52/lilconfig) is a smaller alternative to cosmiconfig for loading,
+object validation libraries exist too.
+
+### keep doing it separately in each tool
+
+It works as it is now, maybe the pain isn't worth the hassle.
+Just add some documentation and raise awareness of potential issues with different implementations
+
+## Unresolved questions
+
+### exact api for validation
+
+How to refactor kit validation to be of general use
+
+### caching
+
+How could we cache the config so that multiple tools being used together can share the same load