`/spawnpoint` cannot be set in the End

Summary:
`/spawnpoint` cannot be set in the End. In Java, this was added in update 1.16.
https://www.minecraft.net/en-us/article/nether-update-java

The spawnpoint command now supports being run in any dimension.

This would be useful for the mapmaking.

Steps to reproduce:

1. Move to the End.

2. /spawpoint

Observed results:
Error: The spawn point cannot be set in this dimension.

Expected results:
Set spawn point at the current coordinates in the End.