Add singlenode mapgen; generates solely the node 'mapgen_singlenode', defaults to air

src/CMakeLists.txt

@@ -227,6 +227,7 @@ set(common_SRCS
 	mapgen.cpp
 	mapgen_v6.cpp
 	mapgen_indev.cpp
+	mapgen_singlenode.cpp
 	treegen.cpp
 	dungeongen.cpp
 	content_nodemeta.cpp