[feat] Add defineMarker() for custom markers for selector combinators

[nmn]

What changed / motivation ?

The previous PR adds support for descendent/sibling selectors but only with a defaultMarker. That works for many common cases, but sometimes more that one marker is needed to differentiate between two ancestors, or two siblings etc.

This PR adds an API for defining custom markers with the defineMarker API.

Additional Context

Needs a little more testing to build confidence. The same value needs to be both a string and an object in different situations, so we need to be a bit careful.

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.

[email protected] compare
node ./compare.js /tmp/tmp.eSuIWGkREO /tmp/tmp.0NnEJ2nFmf

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	622	615	0.99	-
· complex create	200	181	0.91	!!
babel-plugin: stylex.createTheme
· basic themes	455	443	0.97	-
· complex themes	43	42	0.98	-

[github-actions]

workflow: benchmarks/size

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

[email protected] compare
node ./compare.js /tmp/tmp.AEqGQQ5Nn7 /tmp/tmp.A884WOSax9

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,278	1,296	1.01	+
· minified	3,986	4,111	1.03	+
@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