setup eslint & fix some

[dulmandakh]

This PR introduces very basic eslint setup, and GitHub Actions for it. https://github.com/dulmandakh/pm2/runs/4904550547

eslint fixes are not complete yet, but want to have input from others

Q	A
Bug fix?	no
New feature?	no
BC breaks?	no
Deprecations?	no
Tests pass?	yes
License	MIT