Add mod "Grass Slabs, Carpets & Stairs".

This was suggested by the author of FramedBlocks to get nicer textures
on grass slabs. (See XFactHD/FramedBlocks#141 (comment)).

https://www.curseforge.com/minecraft/mc-mods/grass-slabs-carpets-stairs

manifest.json

@@ -1034,6 +1034,11 @@
       "fileID": 5969656,
       "projectID": 1005914,
       "required": true
+    },
+    {
+      "fileID": 5091889,
+      "projectID": 565833,
+      "required": true
     }
   ]
 }