Auto merge of #60586 - cramertj:await, r=oli-obk
Implement built-in await syntax
Adds support for .await under the existing async_await feature gate.
Moves macro-like await! syntax to the await_macro feature gate.
Removes support for `await` as a non-keyword under the `async_await`
feature.
This new syntax is not final, but is the consensus solution proposed by the lang team, as explained in https://boats.gitlab.io/blog/post/await-decision/
Fix https://github.com/rust-lang/rust/issues/51719
Fix https://github.com/rust-lang/rust/issues/51751
Fix https://github.com/rust-lang/rust/issues/60016