Add DYN_REC_STOP to dynamic macros

Dynamic macro functionality is modified to check for `DYN_REC_STOP`, so
that macro recording can be stopped with a designated key combination
(e.g. `qs` or anything) instead of mandating the use of a `_DYN` layer.

`_DYN` layer stopping can still be done by passing `DYN_REC_STOP` within
`process_record_user()`:

    bool process_record_user(uint16_t keycode, keyrecord_t *record) {
      uint16_t macro_kc = (keycode == MO(_DYN) ? DYN_REC_STOP : keycode);
      if (!process_record_dynamic_macro(macro_kc, record)) {
        return false;
      }
      return true;
    }

quantum/dynamic_macro.h

@@ -40,6 +40,7 @@
 enum dynamic_macro_keycodes {
     DYN_REC_START1 = DYNAMIC_MACRO_RANGE,
     DYN_REC_START2,
+    DYN_REC_STOP,
     DYN_MACRO_PLAY1,
     DYN_MACRO_PLAY2,
 };
@@ -209,9 +210,8 @@ bool process_record_dynamic_macro(uint16_t keycode, keyrecord_t *record)
     } else {
         /* A macro is being recorded right now. */
         switch (keycode) {
-        case MO(_DYN):
-            /* Use the layer key used to access the macro recording as
-             * a stop button. */
+        case DYN_REC_STOP:
+            /* Stop the macro recording. */
             if (record->event.pressed) { /* Ignore the initial release
                                           * just after the recoding
                                           * starts. */