Add Prettier config

[RoelN]

I accidentally ran formatting, apparently, which changed all the files to my global config. This PR adds a simple Prettier config so we can be sure the desired formatting is enforced.

[Pomax]

The power of prettier is that it just "does the right thing already" without needing a conf - is your editor set to use a different formatter?

[RoelN]

If you have a config in a directory above (like I have in ~) or have something set up in your text editor, it'll use that. This way you make sure you have deliberate rules for your project's formatting, instead of relying on there being no other config files Prettier considers so it uses its defaults.

[Pomax]

very well. I'm using biome a lot more these days, since it's not written in JS, so it's super fast, but that's overkill for now.