Add luacheck (#3)

Co-authored-by: syimyuzya <syimyuzya@gmail.com>

.luacheckrc

@@ -0,0 +1,17 @@
+unused_args = false
+max_line_length = 180
+
+globals = {
+    "minetest",
+	"signs_lib",
+}
+
+read_globals = {
+	-- Builtin
+	table = {fields = {"copy"}},
+	"ItemStack", "vector",
+	
+	-- Mod deps
+	"intllib",
+	"screwdriver",
+}