Add encoder abstraction.

[tzarc]

Description

Splits out encoder processing logic so that the underlying driver is decoupled from high-level encoder logic.
Effectively allows for "custom encoder lite":

// Driver implements:
void encoder_driver_init(void);
void encoder_driver_task(void);
// Driver invokes during its task:
bool encoder_queue_event(uint8_t index, bool clockwise);

rules.mk picked up:

ENCODER_DRIVER ?= quadrature
VALID_ENCODER_DRIVER_TYPES := quadrature custom

info.json picked up:

{
    /* ... */
    "encoder": {
        "driver": "quadrature" // or "custom"
    },
    /* ... */
}

Quadrature encoder has other weakly-defined functions in order to be able to just override pin reads\s without needing to replicate logic.

Split support is present and tested.

Events are queued and executed on the main loop -- this caters for cases such as split or interrupt-driven, where production of events may not coincide with the main loop's invocation.

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

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).

[zzeneg]

@tzarc I've tested new code for Pica40, all works!

[github-actions]

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[drashna]

Just to match what is in core already