branch: fluent

[jessegreenberg]

A branch for phetsims/joist#992.

It will have classes for managing Fluent.js messages, patterns, and pre-load scripts to load files.

Eventually, these will likely need to be re-implemented into our other string loading scripts and modules. But I couldn't understand how that code worked and changing it incurs more risk and slowdown. If fluent.js is confirmed as the way forward, I can work with a developer that is more familiar with that code to bring it to production.