add luacheck and github workflow (#13)

.luacheckrc

@@ -0,0 +1,18 @@
+unused_args = false
+allow_defined_top = true
+
+globals = {
+    "minetest",
+}
+
+read_globals = {
+    string = {fields = {"split"}},
+    table = {fields = {"copy", "getn"}},
+
+    -- Builtin
+    "vector", "ItemStack",
+    "dump", "DIR_DELIM", "VoxelArea", "Settings",
+
+    -- MTG
+    "default", "sfinv", "creative",
+}