Biome API / cavegen: Add definable cave liquid for a biome (#7192)

Add 'node_cave_liquid' as a new field in biome registration.
If field is absent cave liquids fall back to classic behaviour.

src/mapgen/mg_biome.h

@@ -52,6 +52,7 @@ public:
 	content_t c_river_water;
 	content_t c_riverbed;
 	content_t c_dust;
+	content_t c_cave_liquid;
 
 	s16 depth_top;
 	s16 depth_filler;