Prettier and linting should be consistent across all repos (and agree with each other)

[Peeja]

Note: Given we're planning to move to (a) monorepo(s), this should probably wait for that. It's a lot easier to do this in one (or fewer) repo(s).

Acceptance Criteria

1. Every JS/TS repo uses the same Prettier config.
2. Every JS/TS repo uses the same lint config, at least for stylistic rules2
3. The Prettier and ESLint rules don't conflict.
4. Files can be formatted correctly on save by an editor.
5. Failing to format files breaks CI.
6. We don't depend on hd-scripts, which we have too little control over.
7. Lint warnings fail the build.
   • @Peeja's reasoning:
     • Errors should represent likely incorrect behavior—code that's actually broken.
     • Warnings should represent code which is confusing, less performant than it could be, or otherwise out of line with our house style, but which behaves as intended when run.
     • Either should fail CI, because we don't want to merge either to main.
     • The difference is simply what the linter communicates to the author. In particular, it's more likely to be correct to lint-ignore a warning, while lint-ignoring an error is a signal that something bad may be happening.

[Peeja]

(I've thrown a bunch of things in here which could be broken apart, either for granularity or because we don't all agree on all of them.)