Add prettier to normalize code style

[botandrose]

Okay, @1cg , here's the first sample of what a PR for this might look like. I'm betting you'll have some opinions about this!

Questions:

1. Prettier defaults to 2-space tabs, rather than your 4 spaces. Do you want this to be changed to 4 in the prettier config?
2. Any other config settings you wanna tweak?
3. Turns out it can tidy up Markdown. That seems... nice? Or should we not bother and add *.md it to .prettierignore
4. How should we enforce this, moving forward? It'd be trivial to add a job to the CI to fail the build if prettier wasn't ran, for example. But then we'd need to communicate to PR authors to be sure to run npm run format before committing. That's some minor added friction.

[botandrose]

@1cg Okay, good to go. We discussed in Discord, but for posterity:
1 & 2. Sticking with defaults
3. No markdown, presentation is more important than consistency in docs
4. Not a lot of churn in this repo normally, so will be run manually before release