Added trade state reload command

[cubefury]

This PR adds the command /vending reload tradestate [player], allowing admins to reload a player's trade state while server is running.

• Player argument is optional, will reload tradestate for self if run by a character, and fail with a relevant error message when run on server console.
• Admins who want to reset the tradestate for a player can simply delete the player's tradestate file and run the command.
• Player does not need to be online, their UUID just needs to have been cached from logging in at least once.
• Player does not need to log off while tradestate is refreshed, although they should close out of the vending machine UI before the command is run, since exiting from the UI writes the current trade state to file and may override the desired reload (I'll add a check for this in a future patch to prevent this weird quirk)

Tested in dev env in SP and MP.

[UltraProdigy]

Just one typo really otherwise looks & functions great

[UltraProdigy]

💰