Improve linting & prettier settings

Note: coming from https://github.com/eclipse-thingweb/td-tools/pull/7#issuecomment-1999400383

> I noticed some shortcoming w.r.t. to the current setup.
> * in node-wot we used to have `eslint-plugin-unused-imports`. I think having this might be useful
> * I also noticed that "npm run lint" does not _screem_ if I have unused variables in my code. Useful again I think.
> * I also wonder whether prettier & linting should be based on language level (hence moving it into the `/node` directory )