Feature/eslint

[XanderVertegaal]

Two commits:

• one adds ESLint as a dev dependency
• one fixes all existing warnings/errors

The BWI and iAnalyzer also use ESLint, so you might not object to adding it to Lettercraft as well :)

(I'd also be up for enabling Prettier, but that is more controversial/stringent, so we should discuss that beforehand.)

[XanderVertegaal]

Rebasing hath plunged this PR into chaos. I'll create a new branch and cherry-pick the two commits from this branch that I need.