Implement #[wasm_bindgen(start_async)]

[daxpedda]

Adds a new attribute called start_async, it has the exact same behavior as start but rewrites the function to a non-async version that contains the spawn_local inside it.

The purpose is to allow async fn main() to be used in Cargo binaries. It's similar to what #[tokio::main] and others do.

This could have also been done by adjusting start, but the issue is that it might break some behavior, for example one could rely on the function actually being async, which it wouldn't be after the rewrite. This could be mitigated by adjusting the behavior only for functions called main, but it's still a breaking change I would argue.

Fixes #3076.

[daxpedda]

Alternatively we could introduce a new attribute called main, which differs from start in the sense that it is supposed to be used for the fn main() function.
No changes in functionality to start_async, but more descriptive.

[daxpedda]

After some more experimentation I think there should definitely be a new attribute main that is different from start, supposed to only be used with fn main().

The code generation could be much simpler and there are some limitations around exporting main multiple times that could be alleviated this way.