[eslint-plugin] Add gap/gridGap shorthand expansion to stylex-valid-shorthands

[abhakat]

Expand gap and gridGap shorthands into rowGap + columnGap longhands. Single-value
gap is always expanded (not passed through) because gap is banned by stylex-valid-styles
propLimits on all surfaces. Also adds gridColumnGap → columnGap and gridRowGap → rowGap
as legacy name renames.

What changed / motivation ?

gap is banned by stylex-valid-styles on all Meta surfaces. Without an autofix in
stylex-valid-shorthands, users see an error from valid-styles with no automated path to fix it.
This PR ensures gap (even single-value) always expands to rowGap + columnGap.

• gap: '10px' → rowGap: '10px' + columnGap: '10px'
• gap: '10px 20px' → rowGap: '10px' + columnGap: '20px'
• gridGap → same expansion as gap
• gridColumnGap → columnGap (legacy rename)
• gridRowGap → rowGap (legacy rename)

Linked PR/Issues

Follow-up to #1477

Additional Context

Added tests covering: single values, numeric values, calc(), var(), !important,
comma/slash/3-value CANNOT_FIX edge cases.

Pre-flight checklist

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

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1 Skipped Deployment

Project	Deployment	Actions	Updated (UTC)
stylex	Skipped		Mar 25, 2026 7:02pm

[github-actions]

workflow: benchmarks/size

Comparison of minified (terser) and compressed (brotli) size results, measured in bytes. Smaller is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.20eCZayt5N /tmp/tmp.4pn1gidk6D

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,447	1,447	1.00
· minified	4,558	4,558	1.00
@stylexjs/stylex/lib/cjs/inject.js
· compressed	1,793	1,793	1.00
· minified	4,915	4,915	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	99,653	99,653	1.00
· minified	747,850	747,850	1.00
Done in 0.08s.
Done in 0.32s.

[github-actions]

workflow: benchmarks/perf

Comparison of performance test results, measured in operations per second. Larger is better.
yarn workspace v1.22.22
yarn run v1.22.22
$ node ./compare.js /tmp/tmp.gkPyiIHVE4 /tmp/tmp.VGl6MW25TY

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	533	529	0.99	-
· complex create	66	65	0.98	-
babel-plugin: stylex.createTheme
· basic themes	437	435	1.00	-
· complex themes	34	33	0.97	-
Done in 0.08s.
Done in 0.32s.