Add gather item action

[sammypanda]

• Update docs
• Add ITEMS action option
• Integrate ITEMS option with gather item action
• Create gather items listener
• Hook up gather items listener to completion of the action

[sammypanda]

Did some light testing on e1f0749 and can somewhat confirm it's working fine. Can't confirm if it's broken anything as a side effect, but it shouldn't have.

[sammypanda]

Would be good to see:

• Clearer message/indication that upon adding an item, the default amount is 1.
• Better styling
• Making items optional to gather (like [some eample here])
• Other accepted blocks (like 'gather 5 wood' could be oak, but also allow birch spruce etc)
• ...tbd