Add async generators support via AST transformation

[trossimel-sc]

Summary

This PR introduces support for async generators through AST transformations. In detail:

• Added AsyncGenerator.cpp to transform asynchronous generators into standard generators using AST transformations. This implementation mirrors the logic found in Babel .
• Introduced an EnableAsyncGenerators flag to manage this functionality.

Test Plan

• Added unit tests

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@avp has imported this pull request. If you are a Meta employee, you can view this in D78378721.