infra: auto-format changes with dprint?

[chriskrycho]

I was thinking about how we can make the process of working on the book easier, and one big way is to have format-on-save “just work” in people’s editors using something like Prettier or dprint. (I would pick dprint between those two.) In that case, I was thinking we might also want to set up some automation like the DefinitelyTyped project did, where a bot fixes formatting if it doesn’t match when merging the PR, so that contributors don’t have to try to get formatting done as part of their contribution—e.g. if they make a fix from the web editor.

(This is filed squarely in “ideas that might not be a good fit for us” bucket, but I wanted to write it down here while I was thinking about it so I did not forget it!)