Hotfix: Missing CommonJS convenience files for core build artifacts

Described in #1636 - this includes the addition of lib/core/ CommonJS convenience files, which make it easier to require or import Fine Uploader core builds. The ES6 & CommonJS feature docs page includes updated information that explains how to easily import core files.

I also found some typos in the examples on that doc page that illustrate how to import Fine Uploader directly using a <script> tag. In many cases, one directory entry was missing. That too is now fixed.