Add Luacheck linter CI job

Author: alerque

.github/workflows/luacheck.yml

@@ -0,0 +1,18 @@
+name: Luacheck
+on: [push, pull_request]
+jobs:
+  luacheck:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Checkout
+      uses: actions/checkout@v2
+    - name: Setup Lua
+      uses: leafo/gh-actions-lua@v5
+      with:
+        luaVersion: 5.3
+    - name: Setup Lua Rocks
+      uses: leafo/gh-actions-luarocks@v2
+    - name: Instalal luacheck
+      run: luarocks install luacheck
+    - name: Lint all the Lua code
+      run: luacheck .

README.md

@@ -2,7 +2,8 @@
 
 [![Rust Build](https://img.shields.io/github/workflow/status/alerque/vim-commonmark/Rust?label=Rust)](https://github.com/alerque/vim-commonmark/actions?workflow=Rust)
 [![Rust Code Format](https://img.shields.io/github/workflow/status/alerque/vim-commonmark/Rustfmt?label=Rustfmt&logo=Rust)](https://github.com/alerque/vim-commonmark/actions?workflow=Rustfmt)
-[![Vint](https://github.com/alerque/vim-commonmark/workflows/Vint/badge.svg)](https://github.com/alerque/vim-commonmark/actions?workflow=Vint)
+[![VimL Lint Status](https://github.com/alerque/vim-commonmark/workflows/Vint/badge.svg)](https://github.com/alerque/vim-commonmark/actions?workflow=Vint)
+[![Lua Lint Status](https://img.shields.io/github/workflow/status/alerque/vim-commonmark/Luacheck?label=Luacheck&logo=Lua)](https://github.com/alerque/vim-commonmark/actions?workflow=Luacheck)
 
 Vim syntax highlighting and filetype plugin for [CommonMark][commonmark] using the [pulldown-cmark][pulldown-cmark] parser to inform the parsing. Because the syntax is not cobbled together from regular expressions but informed by a reas parser it will only ever highlight 100% valid CommonMark syntax (no false positives!).