capture Escape key in fullscreen mode

[felix9]

some games/emulators use the Escape key, which is fine, but when running in fullscreen mode on a chrome browser, Escape exits fullscreen mode. pressing escape again will send Escape to the game, and then you can enter fullscreen mode again, but this is awkward and inconvenient.

chrome allows fullscreen apps to capture the Escape key (the user then exits fullscreen by holding down Escape).

details here https://developer.chrome.com/blog/better-full-screen-mode

note, this has to be done by the top-level page. an iframe does not get fullscreenChange events, and it cannot invoke keyboard.lock

this would be a useful option for games embedded on itch.io. (it might be reasonable to do this unconditionally? but an option seems safer.)