add .luacheckrc and fix common errors

.luacheckrc

@@ -0,0 +1,19 @@
+
+globals = {
+	"lightning"
+}
+
+read_globals = {
+	-- Stdlib
+	string = {fields = {"split"}},
+	table = {fields = {"copy", "getn"}},
+	"PcgRandom",
+
+	-- Minetest
+	"minetest",
+	"vector", "ItemStack",
+	"dump",
+
+	-- mods
+	"default", "fire"
+}