[unplugin] Add support for compiling stylex from imported packages

[nmn]

What changed / motivation ?

The unplugin package today has two known issues:

1. The documentation in the Readme is incorrect and misleading
2. It doesn't handle stylex within external packages correctly in a vite project.

This PR attempts to fix both those issues.

For external packages, the plugin will attempt to detect them automatically, but it also lets you pass in a config option as an escape hatch

Next Steps

1. The documentation should be a lot better, but it needs to be verified and rewritten for clarity.
2. Additional testing for building packages that use packages that use StyleX.

[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.HlTywevXTB /tmp/tmp.2EYdDuIQ3U

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	598	624	1.04	+
· complex create	68	67	0.99	-
babel-plugin: stylex.createTheme
· basic themes	451	455	1.01	+
· complex themes	43	42	0.98	-

[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.lA263jI2EX /tmp/tmp.V2AVGK9o21

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,311	1,311	1.00
· minified	4,150	4,150	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

[sfc-gh-alisowski]

Looks great!