[babel-plugin] adjust descendant selector .when priorities

[mellyeliu]

Let's use the following priorities, going roughly from most to least specific. In the longterm sort-keys should be refactored to enforce this order, though we don't yet use that rule internally because of other blockers.

• ancestor
• descendant
• anySibling
• siblingBefore
• siblingAfter

[github-actions]

workflow: benchmarks/perf

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

benchmarks@0.16.1 compare
node ./compare.js /tmp/tmp.TrCgUGJV4C /tmp/tmp.Z43ecZ4V71

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	642	600	0.93	!!
· complex create	175	177	1.01	+
babel-plugin: stylex.createTheme
· basic themes	420	440	1.05	+
· complex themes	41	42	1.02	+

[github-actions]

workflow: benchmarks/size

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

benchmarks@0.16.1 compare
node ./compare.js /tmp/tmp.C4NOwc1MwP /tmp/tmp.i2hc0mjSmI

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

[Prakshal-Jain]

Lgtm