[swift2objc] Support async initializers and properties

[AmrAhmed119]

• Closes [swift2objc] Support async initializers and properties #1778

• Add Support for async getters by converting them into ordinary methods.

• Add Support for async initializers by converting them into static methods.

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[github-actions]

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[liamappelbe]

Looks pretty good. I like that you reused the pattern from the last PR in transform_compound. I just have one smallish comment.

EDIT: Also, looks like you need to run dart format.

[coveralls]

coverage: 81.419% (+0.07%) from 81.352%
when pulling 3982fd1 on AmrAhmed119:async-init-properties
into e62b691 on dart-lang:main.