new sign transaction event

[sumsar1812]

• Added new SignTransactionEvent to notify external plugins about sign transactions
• Fixing checkstyle errors and moving event to aligning with other event location.

Information

The feature request is here:
#6070
https://gyazo.com/ba209d0b4dd4e4a032e1e0a390e37157

This PR closes #6070.

Details

Proposed feature:

This feature would introduce one new event for external plugins to listen to.
This event is called SignTransactionEvent and is fired when a user buys or sells something from a sign.
The event includes information about the type of transaction(buy/sell), the item bought/sold, how much it was bought/sold for and the sign it was bought at, the event is cancelable.

Environments tested:

OS: Windows 10

Java version: java version "17.0.7"

• Most recent Paper version (1.XX.Y, git-Paper-BUILD)
• CraftBukkit/Spigot/Paper 1.12.2
• CraftBukkit 1.8.8

Demonstration:

https://gyazo.com/b3ab1a544ae3da83c9dd84556a7db415 - gif in 1.8
https://gyazo.com/ddf184d52fe0144690a49e19de0b2c40 - works in 1.12
https://gyazo.com/b68c7554c5ceb455aaa14fc6b4827c1b - works in 1.21
Code used to test it
https://pastebin.com/rcDJbX6s

[JRoy]

you need to declare a HandlerList for the SignTransactionEvent and override the getHandlers method + add a static getHandlerList method which also returns it.

otherwise i just left a few slight style/doc changes.

[sumsar1812]

you need to declare a HandlerList for the SignTransactionEvent and override the getHandlers method + add a static getHandlerList method which also returns it.

otherwise i just left a few slight style/doc changes.

Are the handerlist not inherited by the other event ? So it should not be needed to declare it again, otherwise I think the event wouldn't have worked.

[sumsar1812]

I updated the rest though, thanks :)