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

@@ -0,0 +1,10 @@
+/*
+ * Copyright (c) 2020 The ZMK Contributors
+ *
+ * SPDX-License-Identifier: MIT
+ */
+
+#define KT_ROW(item) (item >> 8)
+#define KT_COL(item) (item & 0xFF)
+
+#define RC(row, col) (((row) << 8) + (col))