CI: add lua lint check

We now have our first lua script so we should start checking that for
obvious problems

.github/workflows/vader.yml

@@ -22,6 +22,7 @@ jobs:
         run: pip install --pre vim-vint
       - name: Lint vimscript files
         run: vint --warning --enable-neovim ./after ./autoload ./compiler ./ftdetect ./ftplugin ./indent ./syntax
+      - uses: lunarmodules/luacheck@v1
       - uses: ludeeus/action-shellcheck@master
 
   tests:

.luacheckrc

@@ -0,0 +1,6 @@
+-- vim: ft=lua tw=80
+
+-- Global objects defined by the C code
+read_globals = {
+    "vim",
+}