Create "How to use eslint" for a MagicMirror module (with v8 and v9)

[KristjanESPERANTO]

We recommend using ESLint in the issue list. It would be good if we could write a guide on how to do this in best practice.

As the classic configuration is still widely used, there should be instructions for the classic configuration and also for the planned future configuration (flat config).

I'll try to do this myself soon. Support and feedback is of course also welcome 🙂