[Editor] Better expose EditorDebuggerPlugin.

Now splitted into two classes:
- EditorDebuggerPlugin (RefCounted).
- EditorDebuggerSession (abstract).

This allows the EditorPlugin to be in control of the debugger plugin
lifecycle, be notified when sessions are created, and customize each of
them independently.

We should slowly transition the various profilers and captures in
ScriptEditorDebugger to their own plugins, and decouple
ScriptEditorDebugger from it's UI part (making it the "real"
EditorDebuggerSession potentially dropping the wrappers).

editor/editor_node.cpp

@@ -4174,6 +4174,7 @@ void EditorNode::register_editor_types() {
 	GDREGISTER_CLASS(EditorScenePostImport);
 	GDREGISTER_CLASS(EditorCommandPalette);
 	GDREGISTER_CLASS(EditorDebuggerPlugin);
+	GDREGISTER_ABSTRACT_CLASS(EditorDebuggerSession);
 }
 
 void EditorNode::unregister_editor_types() {