added switching offhand with mainhand via PlayerAction packet (#608)

i added the code to make switching the items in main and offhand work
with the PlayerAction from it.
I tested it and it worked.

crates/valence_inventory/src/lib.rs

@@ -1151,7 +1151,9 @@ fn handle_player_actions(
                     }
                 }
                 PlayerAction::SwapItemWithOffhand => {
-                    // TODO
+                    if let Ok((mut inv, _, held)) = clients.get_mut(packet.client) {
+                        inv.swap_slot(held.slot(), PlayerInventory::SLOT_OFFHAND);
+                    }
                 }
                 _ => {}
             }