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In CPython, `_thread.start_new_thread()` returns an ID that is the same ID
that is returned by `_thread.get_ident()`.  The current MicroPython
implementation of `_thread.start_new_thread()` always returns `None`.

This modifies the required functions to return a value. The native thread
id is returned since this can be used for interop with other functions, for
example, `pthread_kill()` on *nix. `_thread.get_ident()` is also modified
to return the native thread id so that the values match and avoids the need
for a separate `native_id` attribute.

Fixes issue #12153.
Signed-off-by: David Lechner <david@pybricks.com>