[feat] Add an unplugin bundler plugin implementation for various bundlers

[nmn]

What changed / motivation ?

Add a new unplugin package that should hopefully work as a single bundler plugin implementation for various bundlers.

When dealing with multiple bundles, environments, RSCs, dev servers etc, the plugin may or may not work as expected in various bundlers yet. So PostCSS will remain the default recommendation for now. However, we want to start fixing more of those issues over time and improve the devX and compile performance of using StyleX when possible.

It's also a good time to start adding more examples with popular frameworks outside of Next.js that use RSCs and Vite such as Waku, React Router and RedwoodSDK.

Hopefully, we can also migrate away from the old version of Docusaurus eventually and build a better playground on top of this new code.

[github-actions]

workflow: benchmarks/perf

Comparison of performance test results, measured in operations per second. Larger is better.

[email protected] compare
node ./compare.js /tmp/tmp.2IxYC7Jgjy /tmp/tmp.hMLCI8CvCE

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	636	621	0.98	-
· complex create	184	184	1.00
babel-plugin: stylex.createTheme
· basic themes	460	449	0.98	-
· complex themes	42	42	1.00

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.

[email protected] compare
node ./compare.js /tmp/tmp.I16laDoZfY /tmp/tmp.QfRMHDGvMf

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,282	1,282	1.00
· minified	4,025	4,025	1.00
@stylexjs/stylex/lib/cjs/inject.js
· compressed	1,223	1,223	1.00
· minified	3,216	3,216	1.00
benchmarks/size/.build/bundle.js
· compressed	496,650	496,650	1.00
· minified	4,847,840	4,847,840	1.00
benchmarks/size/.build/stylex.css
· compressed	99,853	99,853	1.00
· minified	747,541	747,541	1.00