feat(xanthous): Add a command to describe an item in the inventory

Add a new DescribeInventory command, bound to I, to prompt for an item
in the inventory (anywhere in the inventory, including wielded) and
display a (new) panel describing it in detail. This description includes
the description, the long description, and the item's physical
properties (volume, density, and weight).

Change-Id: Idc1a05ab16b4514728d42aa6b520f93bea807c07
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3227
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/test/Xanthous/DataSpec.hs

@@ -95,4 +95,12 @@ test = testGroup "Xanthous.Data"
              rots
       ]
     ]
+
+  , testGroup "units"
+    [ testGroup "unit suffixes"
+      [ testCase "density"
+        $ tshow (10000 :: Grams `Per` Cubic Meters)
+          @?= "10000.0 g/m³"
+      ]
+    ]
   ]