refactor: replace filename hyphens with underscores

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

app/src/events/keycode_state_changed.c

@@ -5,6 +5,6 @@
  */
 
 #include <kernel.h>
-#include <zmk/events/keycode-state-changed.h>
+#include <zmk/events/keycode_state_changed.h>
 
 ZMK_EVENT_IMPL(keycode_state_changed);