so that we can leverage default keydown behaviors provided by the
browser, such as tabbing to the next input.

The default behavior was the previous behavior, so that this is not a
breaking change.

To make this useful for the tokenizer, we also take steps to keep the
focus on the tokenizer input, unless the input is empty.

We reuse the logic that determines whether "backspace" should clear a
token, or do it's default behavior.