fix(camping): use plain-string minecraft:icon and redraw item icons

The nested {"texture": "..."} form for minecraft:icon is rejected by the
1.21.0 item schema (BDS warned: "this member was found in the input,
but is not present in the Schema"), so tent and hammock had no inventory
icons at all. Switched to the plain-string form used by every other
addon in the repo.

While here, replaced the placeholder PNGs (a flat green triangle and a
red squiggle) with proper 16x16 pixel art:
  - tent: A-frame canvas silhouette with doorway slit and ground line
  - hammock: side-view sling with rope ends rising off-screen and a
    sagging red-and-white striped fabric

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

camping-supplies-addon/camping_supplies_BP/items/hammock.json

@@ -10,9 +10,7 @@
     },
     "components": {
       "minecraft:max_stack_size": 16,
-      "minecraft:icon": {
-        "texture": "hammock_item"
-      },
+      "minecraft:icon": "hammock_item",
       "minecraft:hand_equipped": true
     }
   }

camping-supplies-addon/camping_supplies_BP/items/tent.json

@@ -10,9 +10,7 @@
     },
     "components": {
       "minecraft:max_stack_size": 16,
-      "minecraft:icon": {
-        "texture": "tent_item"
-      },
+      "minecraft:icon": "tent_item",
       "minecraft:hand_equipped": true
     }
   }