Extract the content of the case 'StringTypeAnnotation' into a single … (#34981)

Summary:
This PR extracts the content of the codegen case 'String' into a single `emitString` function inside the parsers-primitives.js file and uses it in both Flow and TypeScript parsers as requested on https://github.com/facebook/react-native/issues/34872. This also adds unit tests to the new `emitString` function.

ref: https://github.com/facebook/react-native/pull/34936

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[Internal] [Changed] - Extract the content of the case 'StringTypeAnnotation' into a single emitString function

Pull Request resolved: https://github.com/facebook/react-native/pull/34981

Test Plan: Run `yarn jest react-native-codegen` and ensure CI is green

Reviewed By: cortinico

Differential Revision: D40376836

Pulled By: cipolleschi

fbshipit-source-id: feb1b07ec7fc2c333f5054f8cd8d18457d985257