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/boards/shields/iris/iris.dtsi

@@ -4,7 +4,7 @@
  * SPDX-License-Identifier: MIT
  */
 
-#include <dt-bindings/zmk/matrix-transform.h>
+#include <dt-bindings/zmk/matrix_transform.h>
 
 / {
 	chosen {