[postcss-plugin] Implement custom importSources for React Strict DOM compatibility

[javascripter]

What changed / motivation ?

This PR adds importSources option to be configurable in order to allow reusing the plugin for react-strict-dom, and sets platform: "web" as babel.caller as required by RSD babel preset.

react-strict-dom currently relies on a community-maintained package postcss-react-strict-dom that I maintain to extract styles statically.

I'm intending to upstream the PostCSS plugin to RSD but the initial attempt required forking the plugin instead of simply re-exporting the plugin with pre-defined config because shouldTransform() function currently skips files that do not match stylex. This leads to additional maintenance burden and is not ideal.

By allowing importSources to be configurable, RSD users can now use this plugin with the following config:

module.exports = {
  plugins: {
    '@stylexjs/postcss-plugin': {
      include: [
        require.resolve('react-strict-dom'),
        // Include source files to watch for style changes
        'src/**/*.{js,jsx,mjs,ts,tsx}',
      ],
      importSources: ['stylex', 'react-strict-dom']
  }
}

After this PR gets merged, on RSD side we can create react-strict-dom/postcss-plugin that re-exports @stylexjs/postcss-plugin so that additional config remains implementation detail for the user.

This strategy seems to be aligned with the strategy necolas is considering for RSD's eslint plugin as well.

Linked PR/Issues

react/react-strict-dom#281 (comment)
react/react-strict-dom#282

Additional Context

I checked this PR makes the plugin compatible with RSD by:

1. Install the plugin from this branch to the examples:

cd react-strict-dom/apps/examples
npm add "file:///PATH_TO_REPO/stylex/packages/@stylexjs/postcss-plugin"

Then, modify postcss.config.js to use this plugin:

module.exports = {
  plugins: {
    '@stylexjs/postcss-plugin': {
      include: [
        require.resolve('react-strict-dom'),
        'src/**/*.{js,jsx,mjs,ts,tsx}'
      ],
      impotSources: ['stylex', 'react-strict-dom']
    },
    autoprefixer: {}
  }
};

The example now renders correctly using the official @stylexjs/postcss-plugin:

npm run dev:web

Pre-flight checklist

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

[javascripter]

Strictly speaking, we only need "stylex" to maintain backward compatibility as that was the default. In other places like eslint plugin, we use @stylexjs/stylex as the default value so I added both @stylexjs/stylex and stylex to reduce confusion.

If we're okay with breaking some backward compatibility, we can just use @stylexjs/stylex to align.

[javascripter]

https://facebook.github.io/react-strict-dom/learn/setup/#babel-configuration
RSD instructs users to configure babel.config.js which extract api.caller(caller => caller && caller.platform), so this just passes the correct value.

[necolas]

LGTM, thanks!