chore: Extract codegen case 'Stringish' into a single emitStringish function

[gabrieldonadel]

Summary

This PR extracts the content of the codegen case 'Stringish' into a single emitStringish function inside the parsers-primitives.js file and uses it in both Flow and TypeScript parsers as requested on #34872. This also adds unit tests to the new emitStringish function.

Changelog

[Internal] [Changed] - Extract the content of the case 'Stringish' into a single emitStringish function

Test Plan

Run yarn jest react-native-codegen and ensure CI is green

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	7,768,561	+0
android	hermes	armeabi-v7a	7,169,658	+0
android	hermes	x86	8,081,830	+0
android	hermes	x86_64	8,053,393	+0
android	jsc	arm64-v8a	9,629,713	+0
android	jsc	armeabi-v7a	8,394,120	+0
android	jsc	x86	9,579,164	+0
android	jsc	x86_64	10,172,202	+0

Base commit: f353119
Branch: main

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
ios	-	universal	n/a	--

Base commit: f353119
Branch: main

[facebook-github-bot]

@rshest has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[react-native-bot]

This pull request was successfully merged by @gabrieldonadel in 305f7c3.

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