Deprecate the `async` opt?

https://github.com/WebAssembly/component-model/pull/646 forbids the async canonopt with non-async functions. This makes the `async` opt to e.g. rust `generate!` look odd since "positive" overrides (like `async: true`) are always no-ops now (right?). "Negative" overrides are still meaningful but it seems like it would be less confusing for those to move to a new opt (`!async: [...]`? 🤷).

Regardless of what is done with the `async` opt in the long run it might make sense to start emitting warnings for positive overrides now, _especially_ `async: true`.

Ref https://github.com/bytecodealliance/wit-bindgen/issues/1622