rust: Highlight async functions in completions (#3095)

Before (code in screenshot is from this branch,
`crates/zed/languages/rust.rs:179`):

![image](https://github.com/zed-industries/zed/assets/24362066/6b709f8c-1b80-4aaa-8ddc-8db9dbca5a5e)
Notice how the last 2 entries (that are async functions) are not
highlighted properly.
After:

![image](https://github.com/zed-industries/zed/assets/24362066/88337f43-b97f-4257-9c31-54c9023e8dbb)

This is slightly suboptimal, as it's hard to tell that this is an async
function - I guess adding an `async` prefix is not really an option, as
then we should have a prefix for non-async functions too. Still, at
least you can tell that something is a function in the first place. :)

Release Notes:
- Fixed Rust async functions not being highlighted in completions.