Initial WebWorker host environment - Pull Request No.2

This work is related to #11750, and is a patch to add a WebWorker hostenv from @ssimpo.

The this is the 2nd pull request (see: https://github.com/dojo/dojo/pull/43 for previous).  I've added the helpful changes suggested by @dylans and @neonstalwart, to try and streamkline this.

All comments and suggestions are very welcome, I will try to make changes where required.