No support for SharedString properties to be get/set

[cl1ents]

I found out the hard way that the Lune does not allow us to either read or set any properties of type SharedString, such as MeshPart.PhysicalConfigData.

There doesn't seem to be any conversion from rbx_dom's SharedString type into a LuaValue, therefor making it impossible for us to do anything with them in a Lune script.