Use peerDependencies

[jackw]

This PR moves (back) to peer dependencies instead of relying on dependencies. According to the previous PR this move away from peerDeps was related to toolkit...

Use dependencies for now
... so that it's not installed to a plugin's node_modules/@grafana/toolkit/node_modules dir but to the parent one where eslint can find it. We can reverse this when we migrate from yarn back to npm (v7+) which installs peerDependencies.

Toolkit is no more and keeping these in sync for grafana/grafana and plugins is a bit of a chore. Peer dependencies should make our lives easier. Seems quite common to use peerDeps for configs:

• airbnb
• alloy
• fb

[abannachGrafana]

I believe this will also match us back up to the README suggesting running npx install-peerdeps --dev @grafana/eslint-config