chore: enable no-unused-vars eslint rule in TS

This MR introduces the no-unused-vars eslint rule for TypeScript files and fixes no-unused-vars errors.

It also adds an npm task clean, which removes the out directory where we keep compiled sources. Automatic fixing of linting errors behaves differently with or without out folder. The reason is that this eslint ignore is or isn't needed depending on whether the out directory has all compiled sources or not.