Biome API / dungeons: Add biome-defined dungeon nodes

Add new biome fields 'node_dungeon', 'node_dungeon_alt', 'node_dungeon_stair'.
If 'node_dungeon' is not defined dungeons fall back to classic behaviour.

Remove messy and imprecise dungeon material code from 'generateBiomes()'.
Code deciding dungeon materials is now in 'generateDungeons()' and uses the
biome at mapchunk centre for more precision.

Remove hardcoded 'MG_STONE' types as long intended.

doc/lua_api.txt

@@ -5676,6 +5676,17 @@ Definition tables
     --  ^ Nodes placed as a blob of liquid in 50% of large caves.
     --  ^ If absent, cave liquids fall back to classic behaviour of lava or
     --  ^ water distributed according to a hardcoded 3D noise.
+        node_dungeon = "default:cobble",
+    --  ^ Node used for primary dungeon structure.
+    --  ^ If absent, dungeon materials fall back to classic behaviour.
+    --  ^ If present, the following two nodes are also used.
+        node_dungeon_alt = "default:mossycobble",
+    --  ^ Node used for randomly-distributed alternative structure nodes.
+    --  ^ If alternative structure nodes are not wanted leave this absent for
+    --  ^ performance reasons.
+        node_dungeon_stair = "stairs:stair_cobble",
+    --  ^ Node used for dungeon stairs.
+    --  ^ If absent, stairs fall back to 'node_dungeon'.
         y_max = 31000,
         y_min = 1,
     --  ^ Upper and lower limits for biome.