feat: lock a momentary layer

[ecstatic-morse]

Resolves #1299.
Resolves #1771.

This adds a momentary layer lock behavior (&molock ) which causes all active momentary layers not to become deactivated on release. If no layers are active, &molock triggers a customizable fallback behavior, which defaults to &to 0.

See the newly added docs for a more complete description.

[ecstatic-morse]

This PR is now feature complete, so I've marked it as ready for review. It would be nice to allow custom momentary layer lock behaviors to accept an arbitrary arguments, but I didn't see any prior art for that and I'm still not too proficient with devicetree/Zephyr, so for now you need to specify all the arguments when the behavior is defined.

[caksoylar]

LGTM from docs POV.

I wasn't sure if a QMK-like dual mode approach where we have "default" layers and regular layers was needed (where such a behavior could convert a layer from normal to default), but this seems to cover the use cases well enough with a simple design.

[ecstatic-morse]

Thanks @caksoylar. The rebased version seems to be working fine (I've been running it since last night).

I've been using this for a few months now and still find it useful as part of my keymap, mostly for locking the navigation layer. I noticed the old documentation was a bit spartan so I added the example. It would be nice to get it upstreamed, but seeing as I'm the only user that seems a bit premature. Maybe I'll ask around on the Discord to see if anyone else wants to try it out...

[Dom324]

Any progress on this? :)
Does the layer lock work also with &lt?

[Townk]

I've been using this feature for some time now, and I think this design is simple, and achieves everything one might need to keep a momentary layer toggled.

BTW, this works with &mo and &lt.

[alternateved]

Great change. I've also been using this for some time and it really works well.