Fixes #24740.
Implements the `uv_mutex_*` and `uv_async_*` APIs.
The mutex API is implemented exactly as libuv, a thin wrapper over the
OS's native mutex.
The async API is implemented in terms of napi_async_work. As documented
in the napi docs, you really shouldn't call `napi_queue_async_work`
multiple times (it is documented as undefined behavior). However, our
implementation doesn't have any issue with this, so I believe it suits
our purpose here.
