Expose getPointedThing to Lua

This commit introduces Raycast, a Lua user object, which can be
used to perform a raycast on the map. The ray is continuable, so one can
also get hidden nodes (for example to see trough glass).

src/script/scripting_server.cpp

@@ -92,6 +92,7 @@ void ServerScripting::InitializeModApi(lua_State *L, int top)
 	LuaPerlinNoiseMap::Register(L);
 	LuaPseudoRandom::Register(L);
 	LuaPcgRandom::Register(L);
+	LuaRaycast::Register(L);
 	LuaSecureRandom::Register(L);
 	LuaVoxelManip::Register(L);
 	NodeMetaRef::Register(L);