Enhancement: Add option to process arbitrary named exports

[nmn]

What changed / motivation ?

By default you import stylex from the @stylexjs/stylex package and the compiler detects and compiles that. It has also been possible to set up custom import paths for custom aliases. However, so far it was expected that you'd still use import * as <name> from a custom source.

This PR adds the ability to mark stylex to be a named export from a custom path. This lets enables the following pattern:

import {html, css} from 'browser';

const styles = css.create({
  foo: {...}
});

css.props(styles.foo);

This will enable larger UI frameworks to incorporate StyleX within their own API.

Additional Context

Added a couple of tests which are passes as expected.

Pre-flight checklist

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

[github-actions]

compressed-size: runtime library

Size change: 0.00 kB
Total size: 2.36 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./packages/stylex/lib/stylex.js	1.02 (3.53)	0.00 (0.00)	0.0% (0.0%)
./packages/stylex/lib/StyleXSheet.js	1.34 (3.41)	0.00 (0.00)	0.0% (0.0%)

[github-actions]

compressed-size: e2e bundles

Size change: 0.00 kB
Total size: 1122.94 kB

View unchanged

Filename: gzip (minify)	kB size	kB change	% change
./apps/rollup-example/.build/bundle.js	998.63 (9939.85)	0.00 (0.00)	0.0% (0.0%)
./apps/rollup-example/.build/stylex.css	124.30 (874.43)	0.00 (0.00)	0.0% (0.0%)

[necolas]

lgtm

[necolas]

please could we cut a release containing this patch? or is there other stuff we're waiting to include?

[nmn]

The Babel plugin bundle occasionally adds a special character. I'm trying to fix that so I can cut a release.