[babel-plugin] refine createTheme and defineVars priorities

[mellyeliu]

Pushed up a bad revision in #1272, re-tweaking priorityForAtRule

Also simplifying some of the logic in createTheme as the default at-rule behaviour was previously forked

[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.xdcFVPMYKE /tmp/tmp.AD1yAxTV2z

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	613	583	0.95	-
· complex create	180	188	1.04	+
babel-plugin: stylex.createTheme
· basic themes	432	440	1.02	+
· complex themes	41	41	1.00

[github-actions]

workflow: benchmarks/size

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

benchmarks@0.16.1 compare
node ./compare.js /tmp/tmp.m8550aInVY /tmp/tmp.Ukzb2WKxMs

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