[Core] Feature: Add Key Cancellation

[Xelus22]

Description

Edit: renamed to Key Cancellation

(Also open to different naming suggestions)

This adds a feature where it is a key interrupt and forces another key to be un-pressed when one key is pressed.

Usage:
keymap.c

#ifdef KEY_CANCELLATION_ENABLE
const key_cancellation_t PROGMEM key_cancellation_list[] = {
    // on key down
    //       |    key to be released
    //       |     |
    [0] = {KC_D, KC_A},
    [1] = {KC_A, KC_D}
};
#endif

This example shows on the first interrupt pair, when D is held, A will be interrupted and unpressed.
The second interrupt pair is when A is held, D will be unpressed.
This means that A and D can never be in the same packet (from a normal keystroke).

Types of Changes

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

Issues Fixed or Closed by This PR

• Add new feature key interrupt

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).

[drashna]

(Also open to different naming suggestions)

As long as it's not snap tap or rappy snappy.

[dexter93]

Some nice graphics for docs can be found here

Oh wait..

[tzarc]

Just some quick cleanup.

[KarlK90]

@Xelus22 Thanks for the PR, here is my first round of review. As the implementation is still changing I only had a look at the test cases.

[tzarc]

That's phase 2 -- @Xelus22 split things up so they were easier to review. There was a WIP branch of his floating about, IIRC.

[dexter93]

renamed aliases to reflect the feature title - however not sure if optimal ( KC also might stand for keycode). Some thoughts from core team?

[tzarc]

Locked as this is deviating from the intent of a PR.