[Feature] - Use Sharables from the Multiverse-Inventories API

[LordTuxn]

Each plot world has its own inventory. Therefore, items must currently be added for each new player who joins the world for the first time.

Using sharables, which can be used by the Multiverse Inventories API, allows us to automatically add items to players with minimal configuration.

An other option would also be to share the inventory between the plot worlds, but I currently don't know how difficulty that is by code.

https://github.com/Multiverse/Multiverse-Core/wiki/API-Usages-(Inventories)