feat(ide-completion): explictly show `async` keyword on `impl trait` methods
OLD:
<img width="676" alt="image" src="https://github.com/user-attachments/assets/f6fa626f-6b6d-4c22-af27-b0755e7a6bf8">
Now:
<img width="684" alt="image" src="https://github.com/user-attachments/assets/efbaac0e-c805-4dd2-859d-3e44b2886dbb">
---
This is an preparation for https://github.com/rust-lang/rust-analyzer/issues/17719.
```rust
use std::future::Future;
trait DesugaredAsyncTrait {
fn foo(&self) -> impl Future<Output = usize> + Send;
fn bar(&self) -> impl Future<Output = usize> + Send;
}
struct Foo;
impl DesugaredAsyncTrait for Foo {
fn foo(&self) -> impl Future<Output = usize> + Send {
async { 1 }
}
//
async fn bar(&self) -> usize {
1
}
}
fn main() {
let fut = Foo.bar();
fn _assert_send<T: Send>(_: T) {}
_assert_send(fut);
}
```
If we don't distinguish `async` or not. It would be confusing to generate sugared version `async fn foo ....` and original form `fn foo` for `async fn in trait` that is defined in desugar form.
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