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/arm/dz60rgb/dz60rgb_rev1.dts

@@ -7,7 +7,7 @@
 /dts-v1/;
 #include <st/f3/stm32f303Xc.dtsi>
 
-#include <dt-bindings/zmk/matrix-transform.h>
+#include <dt-bindings/zmk/matrix_transform.h>
 
 / {
 	model = "DZ60RGB, Rev 1";