Split out async_fn_in_trait into a separate feature

PR #101224 added support for async fn in trait desuraging behind the
return_position_impl_trait_in_trait feature.

Split this out so that it's behind its own feature gate, since async fn
in trait doesn't need to follow the same stabilization schedule.

compiler/rustc_feature/src/active.rs

@@ -312,6 +312,8 @@ declare_features! (
     (active, associated_type_defaults, "1.2.0", Some(29661), None),
     /// Allows `async || body` closures.
     (active, async_closure, "1.37.0", Some(62290), None),
+    /// Alows async functions to be declared, implemented, and used in traits.
+    (incomplete, async_fn_in_trait, "CURRENT_RUSTC_VERSION", Some(91611), None),
     /// Allows `extern "C-unwind" fn` to enable unwinding across ABI boundaries.
     (active, c_unwind, "1.52.0", Some(74990), None),
     /// Allows using C-variadics.