style: missing lint settings

[PartyLich]

Trivial first issue: add a lint config

It's not a huge deal right now, but enforcing consistent code conventions is always a good idea. For example, the current index.js has inconsistent use of semicolons, indents, and quotes. Not a breaking thing, but it's the first thing that jumped out to me when i opened the file.

I'm happy to add an eslint setup, I just don't know what conventions you'd like to adopt. All semis, only essential semis? 2 space indents (the most common js convention ive seen), 4 (to be consistent with the C codebase elsewhere). etc.