add luacheck workflow

.github/workflows/luacheck.yml

@@ -0,0 +1,10 @@
+name: luacheck
+on: [push, pull_request]
+jobs:
+  luacheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@master
+      - name: Luacheck
+        uses: lunarmodules/luacheck@master

.luacheckrc

@@ -0,0 +1,7 @@
+
+read_globals = {
+	"minetest",
+	"ItemStack",
+	"vector",
+	"unified_inventory",
+}