Introduce #[wasm_bindgen(main)]

[daxpedda]

This PR introduces a new attribute for the wasm_bindgen macro: main.
The intention here is to be used on fn main() in a Cargo binary or example.

Currently wasm-bindgen adjusts the main function to be used on startup anyway, but this attribute has a couple of advantages:

• It supports async fn main() in binaries by removing the async from the function signature but leaving the required fn main() otherwise intact.
• Currently returning an error from the main function will do nothing, this adjusts it to actually properly throw the error.
• All return types supported by Rust fn main() are supported, see Termination. This is done with the help of autoref-based specialization, see this case study by dtolnay.

The ui-tests in CI have been adjusted to test on Rust v1.61 to support Termination and it's connected ExitCode, which is why some unrelated ui-test results had to be adjusted.

Replaces #3252.
Replaces #3263.

Fixes #3076.
Fixes #3262.

[daxpedda]

Rebased after #3338.

[daxpedda]

Rebased after #3360.

[Liamolucko]

I'm not quite sure what this line is trying to say? I believe that combining them will actually cause the CLI to throw an error.

[daxpedda]

What I'm trying to say is that using #[wasm_bindgen(main)] doesn't actually call that function when using the init() function. It only works as expected on fn main in a binary or example.

I will try to improve this, English isn't my first language, please feel free to suggest better wording.

[Liamolucko]

You need to add an entry for the guide page to guide/src/SUMMARY.md so that it'll show up in the sidebar.

[Liamolucko]

Er, you didn't actually remove Termination support, you just removed the UI tests and updated the docs.

[daxpedda]

Er, you didn't actually remove Termination support, you just removed the UI tests and updated the docs.

My bad, I forgot to save the file before committing 😆.

[Liamolucko]

Thanks!