`UniqueId` properties should only be written to rbxl and rbxlx files

[kennethloeffler]

Roblox only writes UniqueId properties to place files (rbxl/rbxlx), and never writes them to model files (rbxm/rbxmx). rbx-dom should do this too.

[Dekkonot]

The only obvious answer to this is to add API to distinguish between serializing a place file and serializing a model. I'm not sure what the design of such an API would be though. It feels kinda weird to just have duplicates of all of our public facing API, one for models and one for places.