add eslint

[thenbe]

I'm not really sure if you wanted to add linting, but if you do this might come in handy. As far as strictness goes, it's high up there so feel free to edit where you see fit. Or perhaps you can use it as a reference in CI with a continue-on-error: true.

Run with pnpm run lint.

[CaptainCodeman]

Thanks, this looks very useful.