`/plot grant add/check <name>` doesn't tab complete playerlist

[NotMyFault]

https://github.com/IntellectualSites/PlotSquared/blob/ff83868cbc20f082362bfcb487b62c865a2926e9/Core/src/main/java/com/plotsquared/core/command/Grant.java#L177 returns the playerlist too late. Rather, we need it for each completion (add/check).