Easy way to check `CTRL` `ALT` `SHIFT` etc keys

[BenjaminDRichards]

It would be nice to have properties on game.input.keyboard to quickly check the status of common toggle keys, e.g. game.input.keyboard.isShiftDown.

We can already check this on a per-game basis by setting up listeners for SHIFT-DOWN etc, but this entails an extra interface abstraction layer.

If we do this in-engine, we can also set up some robust handlers around it. For example, if we discover it gets stuck after ALT-TAB or COMMAND-TAB operations - which it might, because the keyup happens outside the browser - we can set up extra listeners for focus etc.

Worth investigating.

[BenjaminDRichards]

Additional: There's currently no easy way to tell whether modifier keys are down at any given moment for pointers. I believe we can only get modifiers at the instance of pointerdown. Easy checks would let us do pointer work without having to build a keyboard abstraction layer.