Add linters

[NicolasCARPi]

A linter for JS like eslint will help you with the codebase. Example config: https://github.com/elabftw/elabftw/blob/master/.eslintrc.js

For instance, it can fix automatically when you use var instead of let/const (I've seen a mixed usage in the js files).

A linter for php like phpcs is of great help to format stuff automatically instead of spending time on trivial issues like where the curly braces go. Example config: https://github.com/elabftw/elabftw/blob/master/.php_cs.dist