[keyboard] Better support for layered keyboards

[pokey]

The way Cursorless keyboard support is built now, someone could just map keybindings to various of our simple keyboard commands. There are a couple things missing with this approach:

• It is a lot less compact than the modal setup. It might be nice to have a way for them to specify the keys as in the modal setup, and we create keybindings with modifiers for them?
• It might be nice to have a setup where first keypress requires modifier, and then until they press an action, they can use normal layer, and when they issue the action it drops back to needing modifier again
• There is an awkward dance where they use modifier to initiate hat selection, then no modifier to select hat, then modifier to issue action