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/dt-bindings/zmk/matrix-transform.h

@@ -4,7 +4,6 @@
  * SPDX-License-Identifier: MIT
  */
 
-#define KT_ROW(item) (item >> 8)
-#define KT_COL(item) (item & 0xFF)
+#warning "matrix-transform.h has been deprecated and superseded by matrix_transform.h"
 
-#define RC(row, col) (((row) << 8) + (col))
+#include "matrix_transform.h"