MDL-XXXXX JavaScript: Add a build process system for JavaScript

This wraps execution and versioning of JavaScript build tools in a grunt
configuration.

This allows us to specify an exact version of build tools for each branch
of Moodle, and to wrap the build processes in common settings suited to how
these should be run within Moodle.

It should be possible to wrap a variety of other tools using grunt,
including less tools and any other relevant systems.