Rename Server `register` API to `extend`
Rename Server `register` API to `extend`

More consistent with how extensions work

renyuneyun/core,datitisev/core,malayladu/core,flarum/core,flarum/core,flarum/core,datitisev/core,Luceos/core,datitisev/core,Luceos/core,renyuneyun/core,renyuneyun/core,malayladu/core,Luceos/core,malayladu/core,datitisev/core,malayladu/core,Luceos/core,renyuneyun/core