Strongholds in the End

Pretty simple, a version of strongholds in the end dimension.

This could be useful for players, as people lost in the End would be able to get back to the overworld without having to find a miniature bedrock gateway. But the main reason I want this is for lore purposes. End portal frames are made of endstone, and endstone comes from the End. So the only way it makes sense is if the portal from the End came first, and endermen used it to get to the overworld. (They later removed most of the eyes from the portals for security reasons.)