Add support for `event.code`

[Ned Schwartz]

While it may prove impossible to smooth over all OS / keyboard layout differences in event.key, providing access to mapping hotkeys to an event.code may help in some situations (for example WASD navigation)

VS Code allows this via wrapping the code in []: for example [KeyA] would match on event.code === "KeyA".

This allows for some flexibility to map shortcuts to physical locations on the keyboard (eg WASD navigation etc.)