Josh Channings

Done. I'm not clear on the difference between `Window` & `UnownedWindow` (in terms of usage), but everything builds & tests the same.

[Demo of upstream PR](https://user-images.githubusercontent.com/16120047/149847320-b29166f8-743f-414c-a714-d435a062a22d.mov)

I'm not sure what the bot is saying here. The version *is* updated vs master in `vcpkg.json`.

Sorry, I ended up just using Nix to bring the dependency in so I don't have a direct motivation to complete the work right now. If I have spare time...

I've just started using Fluent in a project, and thought of a similar way to get compile-time checking. If we generate helper functions for every message, we can also check...