Add `DYN_REC_STOP` to dynamic macros

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;
    }