Option to lock save files to the first map you play them with

### I am planning on implementing this myself

No

### Describe your request

It's surprisingly easy to lose track of which map you were playing a specific save file with.

* If you delete a mod that contains a world map, the save file will automatically switch to a different map
* If you accidentally toggle the world map switcher, you might forget which map you're playing on.

I'd like the option to disable changing the world map after beginning one of them. Switching away from the vanilla map would be allowed only if you haven't proceeded past Prologue.

### Additional context

The save files have a `LastArea_Safe` tag that contains the world map name, so I think it wouldn't be necessary to make any changes to save files to implement this. Just don't allow changing it (except after completing Prologue in vanilla).