Stabilize Path::try_exists() and improve doc

This stabilizes the `Path::try_exists()` method which returns
`Result<bool, io::Error>` instead of `bool` allowing handling of errors
unrelated to the file not existing. (e.g permission errors)

Along with the stabilization it also:

* Warns that the `exists()` method is error-prone and suggests to use
  the newly stabilized one.
* Suggests it instead of `metadata()` to handle errors.
* Mentions TOCTOU bugs to avoid false assumption that `try_exists()` is
  completely safe fixed version of `exists()`.
* Renames the feature of still-unstable `std::fs::try_exists()` to
  `fs_try_exists` to avoid name conflict.

The tracking issue #83186 remains open to track `fs_try_exists`.

compiler/rustc_error_messages/src/lib.rs

@@ -1,6 +1,5 @@
 #![feature(let_chains)]
 #![feature(once_cell)]
-#![feature(path_try_exists)]
 #![feature(rustc_attrs)]
 #![feature(type_alias_impl_trait)]