Lints JavaScript code with ESLint locally and in CI

[willroberts]

This sets and enforces a consistent style for new and existing code. This PR looks ridiculous (300k lines changed...) but I've broken up the commits to make it more reviewable.

I'm making this change now in order to increase our confidence in future changes. This also paves the way to running decaffeinate in order to get all game code using the same style, linters, etc.

Core changes:

• Replaces xojs (an eslint wrapper) with just eslint. Configuration is in .eslintrc.json and .eslintignore (see 038090c).
• Uses the Airbnb JavaScript Style Guide for ESLint rules, with some rules downgraded from error to warn to get things into a clean initial state.
• Lints practically 100% of the repo's JavaScript code. Around 99.5% of these changes were automatically performed by eslint --fix. These changes are grouped in a single commit (63cc664) to make other changes more visible.
• Adds a new linting job in GitHub Actions, which is passing in testing here.

100% of unit tests and an unchanged number of integration tests (87%) are still passing after this change.