Fix some documentation and names

[MrNemo64]

In the description of this entry of FEATURE_NOT_SUPPORTED it is said

It is paramount that plugins ensure that economy providers support certain methods (e.g. bank accounts) before attempting to access them.

yet NonPlayerAccounts are no longer an optional feature

In Acount#setBalance it is said

Specified amounts must be AT OR ABOVE zero.

yet the provider may optionally support negative balances (OptionalEconomyApiFeature#NEGATIVE_BALANCES)

We have 2 packages for common clases between APIs: me.lokka30.treasury.api.common and me.lokka30.treasury.api.misc

There are most likely some more inconcistencies in the code/documentation but these are the ones that I had time to find. We should probably do a full scan of the code after the PR are closed to find these mistakes

EDIT: some more things I found
EconomyFailureReason#ACCOUNT_DELETION_NOT_SUPPORTED but account deletion is not an optional feature
EconomyFailureReason#ALREADY_MEMBER_OF_ACCOUNT and EconomyFailureReason#ALREADY_NOT_MEMBER_OF_ACCOUNT but we no longer have an addMember method, members now are defined by permissions
EconomyFailureReason#ALREADY_OWNER_OF_ACCOUNT and EconomyFailureReason#ALREADY_NOT_OWNER_OF_ACCOUNT we no longer have owners

[lokka30]

Thanks for finding these