Tracking issue for #!feature(async_unsafe)] (RFC 2394)

[Centril]

This is a tracking issue for #![feature(async_unsafe)] (rust-lang/rfcs#2394).
The feature gate provides the async unsafe fn syntax.

Steps:

• Split the feature gate (move async unsafe fn to distinct feature gate #62500)
• Adjust documentation (see instructions on rustc-guide)
• Stabilization PR (see instructions on rustc-guide)

Unresolved questions:

• Is this the right design wrt. calling async unsafe fn?

[Centril]

I was convinced not to feature gate this after all (see #62149 (comment)). Sorry about the noise!