asyncjs: don't use JavaScript `async`

[zerbina]

Summary

• asyncjs no longer uses JavaScript's async
• result in .async procedures now works like it does everywhere else
• methods can no longer use .async

Details

• the goal is removing the usage of the deprecated .codegenDecl pragma
• routines marked with the .async pragma internally use a closure iterator that is then chained to a promise object when awaiting

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To-Do

• [ ] make imported exceptions properly catchable within closure iterators (separate PR)
• [ ] re-add support for returning Futures from .async procedures
• [ ] write a proper commit message

Notes for Reviewers

• prior discussion about this can be found here
• an important prerequisite for the new JavaScript code generator