[entity] Implement support for `target_stopTimer`

Stops defrag timer, when triggered.

## Keys
- [ ] `targetname` | Target Name | targetname |  | Activating trigger points to this.

## Notes
Partially implemented by https://github.com/OpenQuake3/defrag/commit/397d791dd30e85c5c1a9a41ed3c020e48f74d647