Add support for encoder mapping.

[tzarc]

Description

Proof-of-concept: adds encoder mapping, much like electrical matrix mapping.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Keyboard (addition or update)
• Keymap/layout/userspace (addition or update)
• Documentation

Checklist

• My code follows the code style of this project: C, Python
• I have read the PR Checklist document and have made the appropriate changes.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[tzarc]

For discussion RE: #12982 -- intention is to spur discussion about direction.

[drashna]

Can confirm that this works, and on both sides of a split.

This works, but it creates some really, really weird behavior with encoder mapping enabled. On both my oled, and mouse layer stuff (it triggers the "lock mouse on" behavior, though it shouldn't be calling the code AT ALL, and it prints gibberish to the OLED when it ... shouldn't be able to).

Disabling the encoder mapping fixes the issue immediately...

I can no longer reproduce this behavior...

However, it doesn't handle default layers well. It doesn't "fall through" properly.

In fact, it behaves like it's using the keymap, and not the encoder map

[drashna]

Can confirm, on missing layers (eg when using default layers), it's using the keymap instead of the encoder map.