Rollup merge of rust-lang#144870 - Kivooeo:file_prefix-stabilize, r=tgross35

Stabilize `path_file_prefix` feature

This stabilises `Path::file_prefix`, following the FCP in [tracking issue ](rust-lang#86319)

(FCP ended almost a year ago, so if it's needed for proccess we could rerun it)

Closes: rust-lang#86319

Author: Kobzol

tests/missing-test-files.rs

@@ -1,6 +1,6 @@
 #![warn(rust_2018_idioms, unused_lifetimes)]
 #![allow(clippy::assertions_on_constants)]
-#![feature(path_file_prefix)]
+#![cfg_attr(bootstrap, feature(path_file_prefix))]
 
 use std::cmp::Ordering;
 use std::ffi::OsStr;