WASM: Embedded views should lock the keyboard

[zeta0134]

As reported by several users on the NESDev discord, when used as an itch embed, Rustico is allowing keypress events to propagate to the rest of the page, which mostly means attempting to control the game with arrows is causing unwanted scrolling.

We should try to prevent this if we can. There seems to be a keyboard locking API in some browsers, which we could call when the main window is clicked https://developer.mozilla.org/en-US/docs/Web/API/Keyboard/lock

Separately, we may be able to use prevent default when handling the bound keys, which may be somewhat more portable, as the above feature doesn't appear to be supported by Firefox.