[bun-plugin] Add bun plugin support

[kiangtengl]

What changed / motivation ?

Adds a bun plugin so that we can use stylex with bun. Plugin is a wrapper around the stylex babel transforms.

Main features

• Appends to the bundle css file
• Also allows for an option to output to a separate file, i.e. stylex.css

Usage

import stylexPlugin from '@stylexjs/bun-plugin';

await Bun.build({
  entrypoints: ['./src/App.tsx'],
  outdir: './public/dist',
  plugins: [
    stylexPlugin({
      useCSSLayers: true,
    }),
  ],
});

Linked PR/Issues

Fixes #497. Issue was closed because bun plugin didn't support require.resolve(..., options) but this has since been updated

Additional Context

Running the build in bun

stylex/examples/example-bun on  HEAD (b559b8a) [!] via  v20.17.0 
❯ bun run example:build
$ bun run scripts/build.ts
Browserslist: browsers data (caniuse-lite) is 7 months old. Please run:
  npx update-browserslist-db@latest
  Why you should do it regularly: https://github.com/browserslist/update-db#readme
Build succeeded!
Output: public/dist

Unit tests for the plugin

 PASS  __tests__/bun-plugin.test.js
  @stylexjs/bun-plugin
    extracts CSS from StyleX files
      ✓ basic styles (47 ms)
      ✓ multiple styles files (14 ms)
      ✓ complex styles with keyframes and pseudo-selectors (10 ms)
    handles files without StyleX
      ✓ ignores files without StyleX imports (1 ms)
    supports CSS layers
      ✓ wraps CSS in @layer when useCSSLayers is true (5 ms)
      ✓ CSS layers with multiple files (6 ms)
    TypeScript/TSX support
      ✓ transforms TSX files (7 ms)
    plugin configuration
      ✓ exports a function that returns a Bun plugin
      ✓ setup function registers onLoad and onEnd callbacks (1 ms)
    edge cases
      ✓ handles non-existent files gracefully (1 ms)
      ✓ handles empty file gracefully (2 ms)
      ✓ resets state between builds

Test Suites: 1 passed, 1 total
Tests:       12 passed, 12 total
Snapshots:   11 passed, 11 total
Time:        0.355 s, estimated 1 s
Ran all test suites.

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

stylex on  add-bun-plugin via  v20.17.0 
❯ npm run lint

> [email protected] lint
> npm run lint:report -- --fix

> [email protected] lint:report
> eslint . --ext .flow,.js,.jsx,.mjs --fix

stylex on  add-bun-plugin via  v20.17.0 took 6s 
❯ npm run flow

> [email protected] flow
> flow

Launching Flow server for /Users/kiangtengl/dev/stylex
Spawned flow server (pid=52452)
Logs will go to /private/tmp/flow/zSUserszSkiangtenglzSdevzSstylex.log
Monitor logs will go to /private/tmp/flow/zSUserszSkiangtenglzSdevzSstylex.monitor_log
No errors!

Test Results

Package | Result
-- | --
@stylexjs/bun-plugin | ✅ 12 passed
@stylexjs/babel-plugin | ✅ 117 passed, 1 failed (pre-existing on main)
@stylexjs/stylex | ✅ Passed
@stylexjs/eslint-plugin | ✅ Passed
@stylexjs/rollup-plugin | ✅ 3 passed
@stylexjs/postcss-plugin | ✅ 12 passed
@stylexjs/cli | ✅ 13 passed, 3 failed (pre-existing on main)
@stylexjs/unplugin | ✅ 3 passed
@stylexjs/shared | ✅ Passed
style-value-parser | ✅ 322 passed

## Pre-existing Issues (Same on Main)
Issue | Cause
transform-stylex-props-test.js fails | Pre-existing on main
@stylexjs/cli 3 tests fail | Pre-existing on main
typescript-tests fails | react-router dependency incompatibility
Docs build fails | react-router / Docusaurus incompatibility

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