[babel-plugin][legacy] add legacyClassnamesSort config in processStylexRules

[mellyeliu]

Similar logic to #1244. Let's migrate to processStylexRules in steps. We use classnames alphanumeric sort internally, let's start with that to avoid visual regressions.

1. Rollout to processStylexRules with useLegacyClassnamesSort: true and legacyDisableLayers: true
2. Codemod legacy css to bump specificity when needed
3. Rollout with legacyDisableLayers: false (add selector hack for CSS ordering)
4. (stretch) Rollout with useLegacyClassnamesSort: false for size win (nice to have)

The stylesheets with these legacy configs should be identical to what we have internally.

[github-actions]

workflow: benchmarks/perf

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

benchmarks@0.16.0 compare
node ./compare.js /tmp/tmp.2lev6ezZiX /tmp/tmp.NHtqvR24I4

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	632	626	0.99	-
· complex create	192	191	0.99	-
babel-plugin: stylex.createTheme
· basic themes	456	447	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.

benchmarks@0.16.0 compare
node ./compare.js /tmp/tmp.wGJI4XkAr8 /tmp/tmp.ChS1UxbTgC

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