Bulletproof ESLint config

[pdehaan]

Reviewing PR #55 and noticed that I'm not seeing some ESLint errors/warnings locally, despite me injecting some sketchy JS. I think our config is slightly off. Maybe due to nested configs or something else.

Once PR55 lands, I'll take a peek and see if I can figure it out. This seems to give me closer to what I expected (after deleting nested /src/.eslintrc.json config):

/* eslint-env node */

"use strict";

// All Mozilla specific rules and environments at:
// http://firefox-source-docs.mozilla.org/tools/lint/linters/eslint-plugin-mozilla.html

module.exports = {
  env: {
    es6: true,
  },
  extends: [
    "eslint:recommended",
    // list of rules at: https://dxr.mozilla.org/mozilla-central/source/tools/lint/eslint/eslint-plugin-mozilla/lib/configs/recommended.js
    "plugin:mozilla/recommended",
  ],
  overrides: [
    {
      files: "src/**",
      env: {
        browser: true,
        webextensions: true,
      },
    },
  ],
  parserOptions: {
    ecmaVersion: 8,
    sourceType: "module",
    ecmaFeatures: {
      jsx: false,
      experimentalObjectRestSpread: true,
    },
  },
  plugins: [
    "json",
    "mozilla",
  ],
  root: true,
  rules: {
    "babel/new-cap": "off",
    "mozilla/no-aArgs": "warn",
    "mozilla/balanced-listeners": "off",
    "comma-dangle": ["error", "always-multiline"],
    "eqeqeq": "error",
    "indent": ["warn", 2, { SwitchCase: 1 }],
    "no-console": "warn",
    "no-var": "error",
    "no-shadow": "error",
    "no-unused-vars": "error",
    "prefer-const": "warn",
    "prefer-spread": "error",
    "semi": ["error", "always"],
    "valid-jsdoc": "warn",
  },
};

The "valid-jsdoc" rule adds about 40 warnings, so we can probably turn that off for now, unless I can come up with a special config that ignores "Missing JSDoc @returns for function" warnings. Conversely, we could switch to eslint-plugin-jsdoc plugin which exposes more options. But in reality, all I care about now is invalid JSDocs or missing/inaccurate parameters.