[w3c/uievents] Add support for keyboard lock to test page (#248)

It would be helpful to have a toggle on https://w3c.github.io/uievents/tools/key-event-viewer.html for activating navigator.keyboard.lock and full-screen. These could be separate options, but at time of writing keyboard lock only works in full-screen, and I don't see much value in entering full-screen other than to test keyboard lock, so a single toggle would probably suffice.

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