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

@@ -19,6 +19,7 @@ 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
       - name: Install shellcheck
         run: sudo apt-get -y install shellcheck
       - name: Lint shell scripts