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`
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