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/usb_conn_state_changed.h

@@ -0,0 +1,20 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#pragma once
+
+#include <zephyr.h>
+#include <usb/usb_device.h>
+
+#include <zmk/event_manager.h>
+#include <zmk/usb.h>
+
+struct usb_conn_state_changed {
+    struct zmk_event_header header;
+    enum zmk_usb_conn_state conn_state;
+};
+
+ZMK_EVENT_DECLARE(usb_conn_state_changed);