keycode_string(): Format keycodes as strings.

This adds the `keycode_string()` function described in
https://getreuer.info/posts/keyboards/keycode-string/index.html
as a core feature.

quantum/quantum.h

@@ -39,6 +39,7 @@
 #include "keymap_common.h"
 #include "quantum_keycodes.h"
 #include "keycode_config.h"
+#include "keycode_string.h"
 #include "action_layer.h"
 #include "eeconfig.h"
 #include "bootloader.h"