Add GitHub workflow and LuaCheck (#34)

* Add GitHub workflow and LuaCheck

* Fix LuaCheck warnings

* Improve README.md

.luacheckrc

@@ -0,0 +1,19 @@
+unused_args = false
+allow_defined_top = true
+max_line_length = 999
+
+ignore = {
+    "name", "drops", "i",
+}
+
+globals = {
+    "minetest",
+}
+
+read_globals = {
+    string = {fields = {"split", "trim"}},
+    table = {fields = {"copy", "getn"}},
+
+    "vector", "ItemStack",
+    "dump",
+}