Add one-shot keys

[jtroo]

https://github.com/qmk/qmk_firmware/blob/master/docs/one_shot_keys.md

Would need to be added to the keyberon library since this needs to manipulate its state machine.

[jtroo]

TeXitoi/keyberon#93

Feature-complete in keyberon, need to add to kanata. I'll wait until I've added both one-shot and tap-dance to kanata-keyberon though. The design for tap-dance shouldn't be too much different from one-shot.