add luacheck

.github/workflows/luacheck.yml

@@ -0,0 +1,13 @@
+name: luacheck
+on: [push, pull_request]
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@master
+    - name: apt
+      run: sudo apt-get install -y luarocks
+    - name: luacheck install
+      run: luarocks install --local luacheck
+    - name: luacheck run
+      run: $HOME/.luarocks/bin/luacheck ./

.luacheckrc

@@ -0,0 +1,16 @@
+unused_args = false
+
+globals = {
+    "minetest",
+	"signs_lib",
+}
+
+read_globals = {
+	-- Builtin
+	table = {fields = {"copy"}},
+	"ItemStack", "vector",
+	
+	-- Mod deps
+	"intllib",
+	"screwdriver",
+}