Package tooling reorganization and README.md cleanup

[jgrosspietsch]

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• Confirm that cross browser testing has been completed.

• Verify that no errors are present in the GPT console window.googletag.openConsole().

Comments

I changed/cleaned a bunch of this codebase to make it more maintainable. Most of the changes in this PR are from properly reconfiguring ESLint.

Minor changes/fixes:

• Removed the dependencies we had listed as production dependencies: esdoc, esdoc-standard-plugin, and promise-polyfill. These were exclusively getting used as dev dependencies so I moved them there.
• The repo had both a package-lock.json as well as a Yarn.lock which were out of sync with each other. The beginning of the README mentioned using NPM, while Yarn was mentioned more near the end. I saw no reason to use Yarn so I removed the Yarn.lock and updated the package-lock.json.
• Jest tests wouldn't work. I had to add a testURL and upgrade Jest to get the tests to run.
• Fixed the spelling, formatting, and phrasing for a lot of README.md.
• ESLint rules were being set and overridden arbitrarily. If you're going to import the AirBnB rules, just use those. I basically started over with our ESLint configuration. Only one rule override in the config. Anything else can and should be overridden inline.
• The dist directory wasn't in our .gitignore.

Webpack changes:

• Upgraded to Webpack 4.
• Webpack now builds two separate bundles, one use as a bundled library and the other as a UMD to be imported in a script tag. The library bundle uses @babel/plugin-transform-runtime and uses core-js polyfills. The UMD bundle uses @babel/preset-env with core-js polyfills.
• promise-polyfill wasn't being used anyway so I uninstalled it. It's been replaced with core-js Promises.
• Removed redundant babel-core and updated @babel/core.

(This is a draft PR because rigorous cross-browser testing is still needed.)

(Also, I would release this as a major update.)