* Move away from DT_LABEL.
* Move to DEVICE_DT_GET for non-behavior device access.
* Move various drivers to `gpio_spec_dt` and `DT` related macros.
* Remove mcp23017 while at it, since better upstream driver is
available.
* Make it easier to use *just* event payloads by defining the data,
and then having event manager macros generate "wrapper structs"
* Improve is_*/cast_* APIs to hide details of full event struct.
* Create `zmk_event_t` typedef to pass to event handlers.
* Bring event names inline w/ consistent `zmk_` prefix.
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
