* Add optional `hold-trigger-key-positions` hold-tap configuration
* Leverage configuration for decision making around when to trigger
hold decision in hold-taps.
* Add docs for new configuration.
* Tests for the new config/decision logic.
Aligns *.h and *.c to underscore naming convention.
These were kept (with warnings) for backwards compatibility with external boards/shields:
- kscan-mock.h
- matrix-transform.h
They should be removed in the future.
PR: #523
jmding8