Add has_custom_data() to TileData

[chocola-mint]

This PR adds the method has_custom_data to TileData, allowing users to check if a data layer exists or not.

Closes godotengine/godot-proposals#11317

[AThousandShips]

I'd approve but I think this should be added as a method to TileSet as well, as: has_custom_data_layer or has_custom_data_layer_by_name

That method would just be:

return custom_data_layers_by_name.has(p_value);

And then have this method call it directly

[AThousandShips]

The naming is something I think the team should also weigh in on, but this is a good change IMO, needs a review from the 2D nodes team but otherwise this looks good!

[groud]

LGTM. And I think the naming is good, as it's consistent with get_custom_data_layer_by_name.

[akien-mga]

Thanks!