Much of this code hasn't been updated in quite some time and this commit does a
small audit of the functionality:
* Implementation functions now centralize all functionality on a locally defined
`Thread` type.
* The `detach` method has been removed in favor of a `Drop` implementation. This
notably fixes leaking thread handles on Windows.
* The `Thread` structure is now appropriately annotated with `Send` and `Sync`
automatically on Windows and in a custom fashion on Unix.
* The unsafety of creating a thread has been pushed out to the right boundaries
now.
Closes#24442
2e11009978