[Proposal]: Async main codegen update

# Async main codegen update

* Specification: https://github.com/dotnet/csharplang/pull/9826
* Discussion: https://github.com/dotnet/csharplang/discussions/9828

## Summary
[summary]: #summary

We update the codegen for `async Task Main` to allow the compiler to use a separate helper from the runtime when present.

## Design meetings
