introduce .prettierrc

[jaschaurbach]

This fixes #3032

The produced is in fact correct (see MDN).
I do not know why prettier suddenly enforces this rule, I could not find it out.

To tell prettier to NOT enforce this rule I introduced .prettierrc wit one rule:
'trailingComma': 'none'

By enforcing this rule, prettier changes some long used .js which used trailing commas bringing inconsistency to the code.

Now it is all consistent.

[dato]

Thanks for investigating this!

The produced code is in fact correct

Indeed, as of some recent JS version, I think?

Since the trailing commas are correct, I believe this could also fixed on the ESLint side, by issuing:

    "parserOptions": {
        "ecmaVersion": 2017,
     },

in .eslintrc.js (or some more recent version).

I do not know why prettier suddenly enforces this rule

This, I could not figure out either. :(
See a comment below about making the diff smaller.