Thing Model - Next steps

based on discussions in https://github.com/eclipse/thingweb.node-wot/pull/674 a collection of ideas/plans

- [x] move Thing Model code to `td-tools`
* use resolver for `Servient` dependency?
* rename `td-tools` to `wot-tools` (DP: Not sure if this is needed)
- [x] use https://www.npmjs.com/package/json-placeholder-replacer for placeholder code ?
- [x] test composed types (e.g., objects and/or arrays) in tests for placeholder
- [ ] add `implements(td: ThingDescription, tm: ThingModel): boolean` function to check if a `td` correctly implements a `tm`.
- [ ] Todo: test thing-model-helpers in a browser