The `JoinHandle` was added as a newer return value from this
constructor. Both return values accomplish more or less the same thing.
The difference is that the `Sender` return value is triggered right
before the thread ends while the `JoinHandle` is triggered after thread
completion. One was used in multiprocess mode and one in single process
mode. In any case, the `JoinHandle` works fine for both cases.
Testing: Multiprocess isn't tested currently, but I confirmed that the
ScripThread shut down properly in multiprocess mode with this change.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>