Rollup merge of rust-lang#89735 - bjorn3:stabilize_proc_macro_is_available, r=petrochenkov

Stabilize proc_macro::is_available

Tracking issue: rust-lang#71436

The FCP for the stabilization of `proc_macro::is_available` has completed.

Author: matthiaskrgr

proc_macro/src/lib.rs

@@ -61,7 +61,7 @@ use std::{error, fmt, iter, mem};
 /// non-panicking way to detect whether the infrastructure required to use the
 /// API of proc_macro is presently available. Returns true if invoked from
 /// inside of a procedural macro, false if invoked from any other binary.
-#[unstable(feature = "proc_macro_is_available", issue = "71436")]
+#[stable(feature = "proc_macro_is_available", since = "1.57.0")]
 pub fn is_available() -> bool {
     bridge::Bridge::is_available()
 }