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/drivers/kscan/kscan_mock.c

@@ -13,7 +13,7 @@
 
 LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
-#include <dt-bindings/zmk/kscan-mock.h>
+#include <dt-bindings/zmk/kscan_mock.h>
 
 struct kscan_mock_data {
     kscan_callback_t callback;