fix: hydrogen sacs can stack

data/json/items/generic.json

@@ -366,6 +366,7 @@
     "name": { "str": "cluster of gas sacs", "str_pl": "clusters of gas sacs" },
     "description": "This is a cluster of membranous bubbles, each about the size of a grape, retrieved from inside a mutant insect.  They float like tiny helium balloons, and are likely full of a lighter-than-air gas helping the bug to fly.",
     "price": 0,
+    "stackable": true,
     "material": "flesh",
     "flags": [ "NO_SALVAGE" ],
     "weight": "50 g",