Allow WorldCreator to pass custom environment key

[Doc94]

This PR add a new field for WorldCreator called customEnvironmentKey for allow pass a key for dimension_type (LevelStem) for use when the environment its CUSTOM.

This allow for example add a datapack like https://modrinth.com/datapack/islands-overworld-dimension?version=26.1.2 and in a Plugin create a world using that dimension with.

Key environmentKey = Key.key("floating_islands"); // the key of the environment in datapack
WorldCreator worldCreator = new WorldCreator(new NamespacedKey("doc", "floating_islands")).customEnvironmentKey(environmentKey);

World world = plugin.getServer().createWorld(worldCreator);

[iambibi]

Why does it say: Status: Awaiting review when one has already been done?

[Doc94]

Part of this PR was handle in the 26.1 update, the part for call a custom enviroment not sure how handle...
can wait for any moment where this #12119 can revive or just add a custom NamespaceKey in WorldCreator for pass the key of "LevelStem" and use that in case the enviroment is CUSTOM...

[Doc94]

I modify this PR again for handle the last 26.1 updates, now the PR focus in just allow pass a custom key for being used when the environment is CUSTOM, if #12119 comes alive in the future can just deprecate the whole Environment in favor of LevelStem

[Doc94]

@Lulu13022002 i add a way to expose the environment key for use in the copy of WorldCreator... if its better wait for the other PR for expose that info then you can rollback this change... but i feel this PR is ready for the first goal about create worlds with custom environments.

[Doc94]

Rebased for handle the rebuild in 1c67f14