Add more detailed Navigation Debug Visualization

- Adds more customization options to ProjectSettings.
- Displays navregion edge connections and navigation polygon edges in editor and at runtime.
- Majority of debug code moved from SceneTree to NavigationServer.
- Removes the irritating debug MeshInstance child node from NavigationRegion3D and replaces it with direct RenderingServer API.

main/main.cpp

@@ -2389,6 +2389,8 @@ bool Main::start() {
 		}
 		if (debug_navigation) {
 			sml->set_debug_navigation_hint(true);
+			NavigationServer3D::get_singleton()->set_active(true);
+			NavigationServer3D::get_singleton_mut()->set_debug_enabled(true);
 		}
 #endif