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/tests/toggle-layer/normal/native_posix.keymap

@@ -1,6 +1,6 @@
 #include <dt-bindings/zmk/keys.h>
 #include <behaviors.dtsi>
-#include <dt-bindings/zmk/kscan-mock.h>
+#include <dt-bindings/zmk/kscan_mock.h>
 #include "../behavior_keymap.dtsi"
 
 &kscan {