Custom dimensions and worldgen files don't update on /reload

The bug

The changes that you made to dimensions will not be saved. If you're on a singleplayer world you must reload your world, servers require rebooting them. This is exceptionally inconvenient, especially for testing environments.

How to reproduce:

1. Download the attached datapack with a custom dimension mc-187938.zip
2. Join the world
   → The custom:foo dimension appears when typing /execute in
3. Modify or rename the custom dimension
4. Run /reload
   → The dimension still has the old name and properties