A project can have only a single fallback route. #701
Description
With the release of #700. You can map any page as fallback route. And generators will take care of mapping it as fallback route. But any project can have a single fallback route. And this check is not happening inside generators now. So, need to add that validation in the generators.
Check need to e added inside https://github.com/teleporthq/teleport-code-generators/tree/development/packages/teleport-uidl-validator