Register.lua: Throw error if node 'light_source' > core.LIGHT_MAX

Add 'core.LIGHT_MAX = 14' to builtin/game/constants.lua with the intention
to replace misplaced 'default.LIGHT_MAX = 14' in Minetest Game.
Add comment in light.h requiring the constant be changed in both places.
Add lighting bug warning to note in lua_api.txt.
There are hundreds of mod uses of 15 which causes a lighting bug.

builtin/game/register.lua

@@ -127,6 +127,9 @@ function core.register_item(name, itemdef)
 				fixed = {-1/8, -1/2, -1/8, 1/8, 1/2, 1/8},
 			}
 		end
+		if itemdef.light_source and itemdef.light_source > core.LIGHT_MAX then
+			error("Unable to register node: 'light_source' exceeds maximum: " .. name)
+		end
 		setmetatable(itemdef, {__index = core.nodedef_default})
 		core.registered_nodes[itemdef.name] = itemdef
 	elseif itemdef.type == "craft" then