[babel-plugin] fix createTheme at-rule wrapping

[mellyeliu]

Need to wrap ltr string in at-rule after unforking logic in #1274

[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.zpw5KVvy0Z /tmp/tmp.HRiOaOhYER

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	619	616	1.00	-
· complex create	180	190	1.06	!!
babel-plugin: stylex.createTheme
· basic themes	446	443	0.99	-
· complex themes	42	41	0.98	-

[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.PxjI0gqmjX /tmp/tmp.waMrbqkwAw

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