Use Mono script metadata also for class->script mapping

[shartte]

This fixes #27095 by enabling the appropriate CSharpScript's to be reused when the mono module attempts to get a CSharpScript for a given Mono class.
This primarily occurs when one does:

in "MyNodeSubclass.cs":
class MyNodeSubclass : Node { ... }

When new MyNodeSubclass() is used, Godot will now correctly assign the script "res://MyNodeSubclass.cs" to the newly created Node, instead of assigning a script with an empty path (which will not be saved to the Scene).

The script metadata is now kept in-memory in two Map's to enable bi-directional lookup.

The only caveat is that during _update_exports, we need to pre-allocate the native Node before we call the user's constructor. Otherwise the default constructor would call back into C++ to create the native Node, which would trigger a call to ResourceLoader::load for the script that is in the process of being created. This would simply fail. If the managed object is already associated with a native object before the constructor is called, this doesn't happen.

[akien-mga]

CC @neikeq

[akien-mga]

What's the status on this PR?

[akien-mga]

Ping.