add VaultStateChangeEvent

[Grabsky]

Simplified version of PaperMC/Paper#11679. May extend this API further in the future but it is already good enough for use-case described in the discussion thread.

I don't know if I put the implementation in the right place because currently - I'm afraid - the event is also called when Vault.State is changed via API and doing so inside the event can create an indefinite loop if handled incorrectly. I'm not sure what should I do about this.

[Onako2]

Is there a reason behind not creating a PR at Paper?
Just asking because Paper is more wide-spread and would profit more people

[Grabsky]

@Onako2: Is there a reason behind not creating a PR at Paper?
Just asking because Paper is more wide-spread and would profit more people

Linked discussion have not received any feedback ever since it's creation and because Paper is generally bigger, I think this PR might have taken much longer than I'd like to. I agree however that more people could benefit from it but I'm currently unable to implement the full suggestion. When I'm a bit more familiar with how patches and such work, I might PR the full version to the Paper instead (assuming nobody else would get to it before me!). Just wanted to start with Purpur, as it seems smaller and more open for small changes like this one.