Decouple matrix scan from task loop

[zvecr]

Description

If I optimise matrix scanning,
  then RGB animations will be smoother
  then Split keyboard transaction iops increase

Why scan the keyboard matrix for interaction 10k times a second when USB would bottleneck at 1k? In theory, ignoring the case for "internal" keycodes, rate limiting certain actions could provide runtime for other components.

Existing issues:

• custom matrix requires calling back into quantum
  • potential for inconsistencies and broken functionality
• tight coupling between matrix scan and various quantum tasks
• main code pathway unnecessarily nested

Various longer term end goals this feeds into:

• allow rate limiting of various functionality
  • potential for hardware timer to drive components such as matrix scan at a fixed rate

TODO:

• Rename of existing api + adding replacement stubs with same name
  • iron out any edge cases
  • split
    • some tasks currently only run on master - Relocate matrix_scan_quantum tasks #15882
    • handle split_post_init and its race condition? - Relocate matrix_init_quantum content #15953
• remove MATRIX_SCAN_SKIP as its use is purely to validate the decouple
• plan for refactor of legacy api from boards with custom matrix - Remove matrix_init_quantum/matrix_scan_quantum #19806

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).

[stale]

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[zvecr]

Closing as most content is now implemented, and rate limiting will be completed separately.