[babel-plugin] create new defineConsts file extension

[mellyeliu]

Depending on the way users serve their CSS, you may want to declare a separate filepath for constants. For example, when using directory-based bundling, you'd need to include all constants files alongside each folder to have constants work globally.

This PR adds a .const appended variant to the suffix defined in themeFileExtension (.stylex.js by default) so packagers can statically analyze which files contain constants. I considered making this configurable like themeFileExtension, but decided to keep this hardcoded for now for simplicity.

As a follow up, we should update the enforce-extension lint rule to handle defineConsts under the new extension, with a config to allow users to hard enforce the .stylex.const.js extension (ie. no defineConsts in the default theme extension). The default behaviour will be that both .stylex.js and .stylex.const.js will be considered valid extensions for files declaring defineConsts exports.

Also contains a fix to valid-styles to respect user configured themeFileExtension

[github-actions]

workflow: benchmarks/perf

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

benchmarks@0.16.2 compare
node ./compare.js /tmp/tmp.bfL65E3LAH /tmp/tmp.cSugXcjPYP

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	638	647	1.01	+
· complex create	193	188	0.97	-
babel-plugin: stylex.createTheme
· basic themes	464	460	0.99	-
· complex themes	44	43	0.98	-

[github-actions]

workflow: benchmarks/size

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

benchmarks@0.16.2 compare
node ./compare.js /tmp/tmp.X3Sd1zduTu /tmp/tmp.N0HUw7bOjv

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,278	1,278	1.00
· minified	3,986	3,986	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

[mellyeliu]

Tested on example-nextJS for safety;

Compiler error with

import { tokens } from './ConstantTokens.stylex.constss';

Resolves correctly with

import { tokens } from './ConstantTokens.stylex.const';