wip(feat): Pick and choose configuration

[Nuclear-Squid]

Opening this as a WIP, as I’m sure there are kinks to iron out. This is a somewhat basic implementation of this feature request I made a couple of days ago. It adds both pick and choose half layers for the Navigation and Function layers, but also allows to choose which layers you want on the thumb keys, for the hrm and 2tk flavors.

[Nuclear-Squid]

Ok, so I’ve been using this for about a week, and I really like it. I’ve set it up in a way that’s very similar to the current VIM_NAVIGATION option, but with the function layer on LTHUMB_REACH, and replaced the fun-pad for a mirror layer, and been great.

There are a couple of issues, though (basically UX issues):

• Handling thumb keys is a bit complicated. For the navigation layer, I’ve basically merged the two current variants of this layer into one (so if one has a &kp and the other an &mo, I’ve put a &sc in this new layer), which fine for a quick test but not great.
• Some settings (like NUM_LAYER) are there not to describe what you want, but an implementation detail to make the keymap work properly. Ideally, the user shouldn’t have to deal with this.
• The system is a bit more complex now. People who liked the VIM_NAVIGATION option now have to deal with a bunch more config options. We could maybe have presets, but it doesn’t feel that satisfying.

(edit: basically, the PR is a wip because it’s ready for testing, but not ready for merge)

[Nuclear-Squid]

Rebased the branch to include recent changes on main.