Change main world spawn point when appropriate

[SlimeDog]

Spigot 1.15.2 (and historic)
MV 4.1.1-b777 (and historic)

When the (main) world spawn point is changed in Multiverse, other plugins that rely on the MC version of that spawn point are unaware of it, so continue to use the MC spawn point. This is easy for the server operator to do manually

setworldspawn x y z

but is also easy to forget. The simplest unifying solution would be for MV to set the MC spawn point when the MV spawn point is changed for the main world. There are (at least) two ways that the MV spawn point is typically changed:

• /mv set spawn at selected location
• Editing plugins/Multiverse-Core/worlds.yml, followed by /mv reload or server restart

Reference (for example): Brettflan/WorldBorder#134

[SlimeDog]

My skills are insufficient to create a PR. If someone else would like to take that on, I would be happy to test.