Refactor sticky/oneshot keys

[xs5871]

There was a thing that didn't work for me, and once again... as I came to realize, the current implementation based on modified hold-tap mechanism wasn't the best choice in the first place and increasingly difficult to work around.

• complete rewrite, basically from scratch
• rebranded to "sticky keys", because that is unarguably the more sensible name
• no more dependency on hold-tap
• actually helpful comments (I hope)
• works from tap dance, can even stack multiple sticky keys from a single tap dance
• even more tests
• new feature to stack with any other keys until all keys are released

The old oneshot module is still in place, can be cleaned up eventually.

[regicidalplutophage]

Oneshot name is a staple when discussing small keymaps. I don't mind the module being called sticky keys, but can you make it clear the sticky keys are the same as oneshot?

I'm a heavy oneshot user with the implementation almost the same as Callum, so I'm going to test it before approving. One feature I'd like is if a sticky keycode is being held for some time, it starts being treated as a normal keycode that's being held. Current implementation didn't mix well with holdtap, so going to test that too. The reason is modifier+mouse inputs are common in CAD packages and mouse being a different device has no effect on sticky/oneshot keys.

[xs5871]

Here's my argument: The term "one shot mod" is QMK specific as far as I can tell, the term "sticky keys" has been around since the late 80's and is used pretty much everywhere, except QMK. If you're not already familiar with QMK and its tendency for whacky names "oneshot" sounds like a key that can only be used once, and that'd be silly. I conjecture that "sticky key" will more readily convey the functionality to the uninitiated.

Mentioning the alternative name is a good idea. We've also been misspelling "one shot".

All of the problems of the old one shot implementation you mentioned should be fixed and verified by unit tests. There were too many corner cases where they didn't quite do what one would expect.