added Luacheck GitHub action (#210)

* added shields.io elements
* #209 luacheck action
* #209 cleanup to pass luacheck
* added check statuses to readme

.github/workflows/check.yml

@@ -0,0 +1,28 @@
+name: Lua Checks
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - main
+      - latest
+      - devel
+  pull_request:
+    branches:
+      - main
+      - latest
+      - devel
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - name: checkout
+        uses: actions/checkout@v3.5.1
+      - name: Luacheck
+        uses: lunarmodules/luacheck@v1.1.0
+        with:
+          # -a = disable warning for unused arguments
+          # -i 121 = Setting a read-only global variable.
+          # -u 512 = Loop can be executed at most once.
+          # -i 542 = An empty if branch.
+          args: . --no-max-line-length -a -i 121 512 542 --exclude-files ./lockbox/* ./*/config.lua --globals _HOST term fs peripheral rs bit parallel colors textutils shell settings window read periphemu http os