Extract FlexLayoutProcessor to a standalone module

[lojjic]

Other folks are wanting to do flexbox layout using Troika's text implementation, but they run into the same problems I did for troika-3d-ui with the async nature of the text layout and Yoga's need for measureText to be synchronous.

It occured to me that FlexLayoutProcessor.js is very close to being standalone, and could be exposed on its own to let people just call requestFlexLayout() with their own style tree. It would run in the worker alongside the text processor, have access to text measurement, and users could apply the result to their main thread objects.

I'm thinking this has a dependency on #47, and we'd create a new package troika-flexbox-layout or similar that just has the flexbox processing code and no other troika framework deps.