Code style – ESLint/Prettier/editorconfig

[macintoshhelper]

I think it may be worth deciding on an ESLint setup and using .editorconfig. Prettier seems to be bundled in with Gatsby, but I'm not a big fan of it personally, especially if it's not running in a pre-commit or in CI. But might be simplest just to agree on a prettierrc config, run it once throughout all the code, and enforce it in PRs.

Gatsby's prettierrc doesn't allow semicolons, which I'd suggest changing.