Apply prettier to css and javascript files

[ulgens]

Coming from #238 (comment)

[ulgens]

@bmispelon Not sure how to proceed with this one, waiting for your input.

[bmispelon]

Thanks for adding this!

I don't think output.css should be reformatted, since it's compiled from the files in the scss directory (I think that means we can keep ignoring the css files in the prettier hook).
Similarly with webfont.js: from what I understand it's a vendored library that we just copied in to the project, so I would rather not touch it.

But everything else I'm up for autoformatting 👍🏻

[bmispelon]

django/djangoproject.com#1838 added a prettier config for djangoproject.com, I suggest we adopt the same rules here

[ulgens]

@bmispelon #248 🎈