Add Navigation function to get all navigation maps

Added new function that returns all created navigation map RIDs from the NavigationServer. The function returns both 2D and 3D created navigation maps as technically there is no distinction between them.

(cherry picked from commit c0fed1d4e8)

servers/navigation_2d_server.h

@@ -53,6 +53,8 @@ public:
 	/// MUST be used in single thread!
 	static Navigation2DServer *get_singleton_mut() { return singleton; }
 
+	virtual Array get_maps() const;
+
 	/// Create a new map.
 	virtual RID map_create() const;