Provide warning when using polygon shapes in CollisionShape2D node

`ConvexPolygonShape2D` and `ConcavePolygonShape2D` are only meant to be
used directly in code and not in the editor for physics-based use cases
specifically.

Developers are advised to use `CollisionPolygon2D` instead, which does
generate those shapes under the hood, handling polygon convexivity,
proper orientation etc.

(cherry picked from commit dc446203be)

editor/plugins/collision_shape_2d_editor_plugin.cpp

@@ -249,11 +249,11 @@ void CollisionShape2DEditor::commit_handle(int idx, Variant &p_org) {
 		} break;
 
 		case CONCAVE_POLYGON_SHAPE: {
-
+			// Cannot be edited directly, use CollisionPolygon2D instead.
 		} break;
 
 		case CONVEX_POLYGON_SHAPE: {
-
+			// Cannot be edited directly, use CollisionPolygon2D instead.
 		} break;
 
 		case LINE_SHAPE: {