[eslint-plugin] add legacyAllowMixedExports config to enforce-extension

[mellyeliu]

We use mixed exports within .stylex.js files because the internal lint rule is too permissive. Mixed exports can be problematic because they pollute the "only imports from .stylex.js files" mental model. It'll also introduce complications for other features we want to build moving forward. The plan is to migrate WWW to this lint rule and turn off this config after migration.

External users should not use this config.

[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.3DmSq8YhpD /tmp/tmp.UiNvrdNlXN

Results	Base	Patch	Ratio
babel-plugin: stylex.create
· basic create	616	610	0.99	-
· complex create	82	177	2.16	!!
babel-plugin: stylex.createTheme
· basic themes	459	433	0.94	!!
· 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.Q5Y0XvbUdf /tmp/tmp.W2IKQvL0Pa

Results	Base	Patch	Ratio
@stylexjs/stylex/lib/cjs/stylex.js
· compressed	1,282	1,282	1.00
· minified	4,025	4,025	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