Start using ESLint for adblockpluscore repository

[kzar]

Background

With #3692 we're adding a base ESLint configuration to the buildtools repository. Once that is finished we want to start using the configuration for the adblockpluscore repository.

What to change

• Add a ESLint configuration to adblockpluscore which inherits from our base configuration:

  {
    "extends": "eslint-config-eyeo",
    "root": true,
    ...
  }
  

• If necessary add an ESLint ignore file to ignore third party libraries such as jQuery.
• Make the necessary changes to the adblockpluscore code so that linting passes.
• Add a note to the README explaining how to lint the code (eslint *.js chrome lib test).

Notes

• Since we're now using ​String.prototype.includes we'll need to bump the minimum Firefox version to 40 when we update the adblockpluscore dependency to include these changes.
• We've changes Cu.import calls to use the alternative syntax, this will require changes in the adblockpluschrome compat code. For example Cu.import("resource://gre/modules/XPCOMUtils.jsm"); becomes const {XPCOMUtils} = Cu.import("resource://gre/modules/XPCOMUtils.jsm", {});.

[abpbot]

A commit referencing this issue has landed:
​Issue 4878 - Start using ESLint for adblockpluscore