Switch to ESLint for all code linting

[Mr0grog]

This switches from JSHint and JSCS to ESLint. My primary goal here was to solve several security vulnerabilities, but I also discovered some places previous rules were not being applied (!). This attempts to follow the same settings the project previously had for JSCS and JSHint.

Fixes #85 and also fixes all security warnings when running npm install 🎉

~Note: this is a draft because ESLint picks up on some things the previous linter was missing, and I want to see the errors get logged nicely in CI before fixing them.~ (Edit: this is done.)

[Mr0grog]

FYI, you can see the errors listed out in the CI logs, but since we are on GitHub actions, it’ll also list the errors right in the “files changed” tab of the PR. 😄

(Usually these will be on lines that were changed, but just not on this PR since it’s finding issues that weren’t previously enforced correctly.)

[Mr0grog]

I’m going ahead and merging this since it’s been sitting for a while, seems safe, and should be particularly helpful with future PRs.