Provide move constructor/assignment operators for better interop with STL

Since it's not possible to expose godot's HashMap, OrderedHashMap, Vector, etc. to GDNative without workarounds, I use STL containers when I want a statically typed container while doesn't need to use it directly with godot.

However `String` only has copy constructors, which causes some problems with STL containers:
Generally having only the copy constructor is not a problem, since STL containers can be used with copy-only types. But it becomes problematic when having something like `std::map>` -- as `std::map` requires either both to be movable, or both to be copyable (at least in practice trying to use the former mentioned type causes compile errors on MSVC).