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menuitem( index, [label,] [callback] )
Adds a custom item to the PICO-8 menu.
* index
The item index, a number between 1 and 5.
* label
The label text of the menu item to add or change.
* callback
A Lua function to call when the user selects this menu item.
To remove a previously added item, call menuitem(i)
where i is the index of the item to remove.