Merge "/give" with "/item"

"/give" should be merged with "/item" just like "/replaceitem" was, so that you can give an item from a source with something like "/item give ... from". It would also give us the option to add an item to a container ("/item give block ...") without specifying the slot, which would be very useful for map-makers.

In my PVP game, I used a dynamic hotbar configuration system to control which items go to which slots using scoreboard values. "/item replace ... from" has been extremely useful for this. However, I made it so that if a slot is occupied, it uses "/give" instead... however, there is no "/give ... from" like there is with the "/item" command, so I had to find another workaround.

Syntax:

item give (block|entity) (<pos>|<targets>) item <item> [<count>]
item give (block|entity) (<pos>|<targets>) from (block|entity) <source> <sourceSlot> [<modifier>]