ESLint cleanup

[raphinesse]

• Add proper ESLint configuration
• Fix ESLint violations instead of ignoring them
• Remove unused function
• Remove node-only code
  • This was previously required for being able to run the tests on Node.js. But since we now run them in an actual browser, this isn't required anymore.
• Check that we only use ES5 in our browser code
  • Required for Android 4.4 (cordova-android) and IE 11 (cordova-windows) support

[raphinesse]

I've updated my changes to not use ES6 or beyond and furthermore add a linter check to avoid such mistakes in the future.

[erisu]

Everything is OK but only had one question and concern that is not blocking for this repo.

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Q: Is there a reason why the test/build.js file was moved outside of the testing directory structure?

IMO: Since cordova-js is not an npm package deployed publicly, maybe this is OK.

Concern: If I saw this in a repo that was deployed as an npm package though, I would find it as a tedious task to ensure that all testing files (spread though out src code) were not deployed with the production package. (.npmignore list)

[raphinesse]

Q: Is there a reason why the test/build.js file was moved outside of the testing directory structure?

Yes, the reason was that it is not a test but a build-tool and the linting rules in build-tools were a fit for it. That being said, I see your concern and I'm not 100% happy with the move either. But I also didn't want to have three different kinds of linting rule files in test/.