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/include/zmk/events/activity_state_changed.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <zephyr.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 #include <zmk/activity.h>
 
 struct activity_state_changed {

app/include/zmk/events/battery_state_changed.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <zephyr.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 
 struct battery_state_changed {
     struct zmk_event_header header;

app/include/zmk/events/ble_active_profile_changed.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <zephyr.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 #include <device.h>
 
 #include <zmk/ble/profile.h>

app/include/zmk/events/keycode_state_changed.h

@@ -9,7 +9,7 @@
 #include <zephyr.h>
 #include <dt-bindings/zmk/modifiers.h>
 #include <dt-bindings/zmk/hid_usage_pages.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 #include <zmk/keys.h>
 
 struct keycode_state_changed {

app/include/zmk/events/layer_state_changed.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <zephyr.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 
 struct layer_state_changed {
     struct zmk_event_header header;

app/include/zmk/events/modifiers_state_changed.h

@@ -8,7 +8,7 @@
 
 #include <zephyr.h>
 #include <zmk/keys.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 
 struct modifiers_state_changed {
     struct zmk_event_header header;

app/include/zmk/events/position_state_changed.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <zephyr.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 
 struct position_state_changed {
     struct zmk_event_header header;

app/include/zmk/events/sensor_event.h

@@ -7,7 +7,7 @@
 #pragma once
 
 #include <zephyr.h>
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 #include <device.h>
 
 struct sensor_event {

app/include/zmk/events/usb_conn_state_changed.h

@@ -9,7 +9,7 @@
 #include <zephyr.h>
 #include <usb/usb_device.h>
 
-#include <zmk/event-manager.h>
+#include <zmk/event_manager.h>
 #include <zmk/usb.h>
 
 struct usb_conn_state_changed {