Add onPlayerJoin functions and permission level fields

[CamperSamu]

This functionality aims to make DemoBox (even) more flexible by allowing a function to be ran for every player that joins the DemoBox.

This can be useful if for example I want to make a temporary creative world for the player to exeriment with.

This PR also removes the currently useless Client hook.

[MattiDragon]

What's with the sudden interst in demobox? Is there some event that I'm unaware of using it? I'll take a look at merging this a bit later

[CamperSamu]

A new ModFest just got announced, but idk if it's for that.
As of me, I am working on a Fabric server and DemoBox is a pretty cool (and simple) piece of software that can be pretty useful for a plethera of reasons!