Add a `$paths` field for folders

[Nolan-O]

Not sure if this is an acceptable feature but I wanted to see if it was possible 🙂

Allows a roblox folder's contents to be the sum of multiple flesystem folders' contents. Enhances the ability for rojo to change a project's form based on project files. E.G. multi-place games, libraries with tiered features for releases, a tests.project.json file

This feature may conflict with syncback since it introduces an ambiguity where new contents of a roblox folder should be stored. If possible, I think it would be acceptable to use the first path in the list of paths as a "dominant" one, treating it like the base folder on the filesystem.

Notes:

• $paths is mutually exclusive with $path
• $paths can only be on Folder nodes (maybe unnecessary?)
• $paths can only point to folders
• $paths ignores the metadata files in those folders (because none of them are the sole instance being synced)