Auto merge of #96881 - est31:join_osstr, r=dtolnay

Implement [OsStr]::join

Implements join for `OsStr` and `OsString` slices:

```Rust
    let strings = [OsStr::new("hello"), OsStr::new("dear"), OsStr::new("world")];
    assert_eq!("hello dear world", strings.join(OsStr::new(" ")));
````

This saves one from converting to strings and back, or from implementing it manually.

This PR has been re-filed after #96744 was first accidentally merged and then reverted.

The change is instantly stable and thus:

r? rust-lang/libs-api `@rustbot` label +T-libs-api -T-libs

cc `@thomcc` `@m-ou-se` `@faptc`

library/std/src/lib.rs

@@ -241,6 +241,7 @@
 #![feature(intra_doc_pointers)]
 #![feature(lang_items)]
 #![feature(let_chains)]
+#![feature(let_else)]
 #![feature(linkage)]
 #![feature(min_specialization)]
 #![feature(must_not_suspend)]
@@ -301,6 +302,7 @@
 #![feature(toowned_clone_into)]
 #![feature(try_reserve_kind)]
 #![feature(vec_into_raw_parts)]
+#![feature(slice_concat_trait)]
 //
 // Library features (unwind):
 #![feature(panic_unwind)]