When looking up a symbol from a library, previously an error was
shown when the symbol did not exist. That caused confusion when the
lookup was completely optional.
This adds a new parameter to that method so that those errors can
be handled manually if needed.
This adds GDNative as a separate class type.
It can be used to interface with native libraries by
using "native calls", which can be registered by modules
(and in future other GDNative libraries?).
It also reworks the currently called "GDNativeScript" into a
"NativeScript" that just makes use of the new GDNative instead
of it being the component that implements that functionality.
