[feature] Add stylex.env for compile time constants

[nmn]

What changed / motivation ?

Partially implements #1358

StyleX already supports stylex.defineConsts but this has a few limitations and trade-offs:

• It can only store individual values. It cannot store objects with different values for different pseudo-classes and media queries for example.
• It generates CSS variables at compile time, which are then inlined at bundle-time. This means it generates different className hashes than what would've been generated if the value was used directly.

This PR introduces a new feature called stylex.env. This is fundamentally a very simple feature that is similar to how process.env is already used in many applications. It allows you to configure the stylex babel plugin with constants that are inlined before the stylex compiler runs. It can also contain arbitrary values.

The main trade-off here is that since this is something that is part of the babel config, every change to the constants provided will invalidate the entire cache of stylex compilation.

Additional Context

Some basic tests have been included. Will add more tests with more advanced use-cases.

Pre-flight checklist

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

[github-actions]

workflow: benchmarks/perf

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

benchmarks@0.16.3 compare
node ./compare.js /tmp/tmp.7a961Wr1QB /tmp/tmp.razEDnEJ92

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	642	629	0.98	-
· complex create	69	69	1.00
babel-plugin: stylex.createTheme
· basic themes	464	457	0.98	-
· complex themes	43	43	1.00

[github-actions]

workflow: benchmarks/size

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

benchmarks@0.16.3 compare
node ./compare.js /tmp/tmp.HI98H2llcF /tmp/tmp.t4DpFyUZO5

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,282	1,310	1.02	+
· minified	4,025	4,084	1.01	+
@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

[mellyeliu]

Some test failures. Let's also discuss whether shareable functions will fit in here as well or under a different API