[fix] Compile named legacyMerge import to match types

[nmn]

What changed / motivation ?

The old legacy function stylex() was replaced by legacyMerge in the @stylexjs/stylex package a while ago, but the Babel transform continues to compile the old default export and does not compile legacyMerge.

This PR, will pre-compile legacyMerge when possible, improving performance for those using this deprecated API.

[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.upYhAjGsNs /tmp/tmp.Zdp5JA8KiX

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	531	520	0.98	-
· complex create	64	63	0.98	-
babel-plugin: stylex.createTheme
· basic themes	437	427	0.98	-
· complex themes	33	33	1.00

[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.Y0Bz4YfRzF /tmp/tmp.zQ0QPFfQwm

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,311	1,311	1.00
· minified	4,150	4,150	1.00
@stylexjs/stylex/lib/cjs/inject.js
· compressed	1,762	1,762	1.00
· minified	4,873	4,873	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