feat(behaviours): Layer Locking (&llck/&layer_lock) behaviour

[ystepanoff]

This PR implements the layer locking behaviour (&llck / &layer_lock) that allows to lock currently active layer(s). It is possible to lock multiple activated layers at once, which allows this to work nicely with conditional layers.

Manual testing I performed:

• [x] locking a layer
• [x] locking a conditional layer

Added an automated test which is also included in this PR. Documentation section for the new behaviour have also been added.

PR check-list

• [x] Branch has a clean commit history
• [x] Additional tests are included, if changing behaviors/core code that is testable.
• [x] Proper Copyright + License headers added to applicable files (Generally, we stick to "The ZMK Contributors" for copyrights to help avoid churn when files get edited)
• [x] Pre-commit used to check formatting of files, commit messages, etc.
• [x] Includes any necessary documentation changes.

[omulh]

It seems this is the same or similar to #1984 and #2717

[ystepanoff]

It seems this is the same or similar to #1984 and #2717

Yep, I raised this PR as it seems #1984 needs some more changes (I'm not implying mine doesn't 🙂), just worried that last changes were pushed about a year ago IIRC). #2717 is slightly different (but serves the same purpose it seems): it adds locking property to the layer behaviours. Regardless of which PR will be chosen I am quite keen to see layer locking in upstream.

[nmunnich]

Closing as #2717 is now in main.