Add initial environmental sounds mod with flowing water sounds

default:river_water_source can also create sound if desired as
rivers are considered to be flowing water.
A simple mod for now, with the intention to later use new engine
environmental sound features if/when they appear.

minetest.conf.example

@@ -64,3 +64,7 @@ default:torch 99,default:cobble 99
 # starting biome, is used.
 # Default value is false.
 #engine_spawn = false
+
+# Whether river water source nodes create flowing sounds.
+# Helps rivers create more sound, especially on level sections.
+#river_source_sounds = false