add luacheck and fix common errors

also fixes https://github.com/pandorabox-io/pandorabox.io/issues/493

.luacheckrc

@@ -0,0 +1,21 @@
+globals = {
+	"hangglider",
+	"areas"
+}
+
+read_globals = {
+	-- Stdlib
+	string = {fields = {"split", "trim"}},
+	table = {fields = {"copy", "getn"}},
+
+	-- Minetest
+	"minetest",
+	"vector", "ItemStack",
+	"dump", "dump2",
+	"VoxelArea",
+
+	-- deps
+	"wardzones",
+	"minetestd",
+	"player_monoids"
+}