feat(web): Provide a numberpad layer that embedded apps can switch to

While reviewing #1218, @mcdurdin writes
> I would like to see us providing a numeric keyboard that is just a number pad layout at some point in the future.

This would be a special keyboard layer included in Keyman Engine, so not visible through the keyboard switcher etc, but embedded apps can switch in for number pad scenarios (e.g. entering phone number)