Allow new prefabs to be easily registered

[vocksel]

Currently when you create a new model you have to manually tag it and add it to the folder where all your prefabs are.

This should be streamlined to allow the user to press a key or use a plugin button to quickly register the prefab. Registering would be a simple TextBox prompt to give it a name for the tag, and off it goes.

[vocksel]

With #9 being integrated, this comes with a change to how the folders are laid out. Everything is retroactively created in ServerStorage, and prefabs are required to be put in a subfolder. This makes adding prefabs a bit trickier, so having a way to automatically add them there and get them tagged is really important.

[vocksel]

We now have public API under _G for doing this. _G.prefabs.register(model, name) and _G.prefabs.registerSelection(name). The latter is the same thing as the former, but implicitly passes in the first selection as the model.

This allows us to easily register a prefab, which will be tagged and cloned to ServerStorage. The original model gets to stick around as well, so you don't have to clone anything back into the workspace to use your newly setup prefab.