(Java) Add a more reliable replacement for portal-based chunk loaders

With the removal of spawn chunks in snapshot 25w31a, we now have three supported ways for players to load far away chunks: ender pearls, portal chunk loaders, and the /forceload command. But in the case of a standard SMP server, where most players do not have operator permissions, none of these options are quite ideal.

Ender pearls only load chunks when their owner is online, so there's no way to load chunks without needing a specific player online.

Portal chunk loaders are quite janky, and certain designs tend to break between game versions. They also load twice as many chunks as are usually needed, since they load the other end of the portal.

The /forceload command is only accessible by players with operator permissions, which makes it infeasible for use on larger SMP servers. It's also easy to forget that you're forceloading a chunk, since there's no visual indicator.

I think the game could benefit from a built-in feature for SMP players to load chunks in one dimension/location indefinitely. Portal chunk loaders are the best existing option for this use case, but have a few shortcomings, and are not always guaranteed to work in future updates. If, for example, we had a block that did the job of a portal chunk loader, it would have less of a performance hit, and would be supported in future updates. Furthermore, it would allow server owners to more easily moderate chunk loading, as there could be a game rule that globally limits or disables these blocks.