Makefile: dependency-driven target to build node_modules
Makefile: dependency-driven target to build node_modules

- added `node_modules` as new target
- dependency on `package.json` and `yarn.lock` allows for quick `make
  node_modules` after a branch change, which noops when the deps have
 not changed
- also added `clean` target

grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana,grafana/grafana