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/keymap.c

@@ -14,10 +14,10 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 #include <drivers/behavior.h>
 #include <zmk/behavior.h>
 
-#include <zmk/event-manager.h>
-#include <zmk/events/position-state-changed.h>
-#include <zmk/events/layer-state-changed.h>
-#include <zmk/events/sensor-event.h>
+#include <zmk/event_manager.h>
+#include <zmk/events/position_state_changed.h>
+#include <zmk/events/layer_state_changed.h>
+#include <zmk/events/sensor_event.h>
 
 static zmk_keymap_layers_state _zmk_keymap_layer_state = 0;
 static uint8_t _zmk_keymap_layer_default = 0;