Keep selected files when regening world

[Valtn]

Describe the feature

PaperMC now places a paper-world.yml file inside each world's directory:

world_nether/
├── DIM-1/
├── level.dat
├── paper-world.yml <-- new
├── session.lock
└── uid.dat

Announcement: https://forums.papermc.io/threads/paper-1-19.344/

It would be good to make /mv regen not lose that file.

How is the feature useful to all Multiverse users?

As people upgrade to 1.19 and start using /mv regen they will most likely want to keep their world customizations.

Agreements

• I have searched for and ensured there isn't already an open issue regarding this.