Add actions to run shellcheck

Author: yankeexe

.github/workflows/main.yml

@@ -0,0 +1,30 @@
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+name: "Shellcheck"
+env:
+  VERSION: v0.8.0
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install shellcheck
+        run: |
+          wget -q https://github.com/koalaman/shellcheck/releases/download/$VERSION/shellcheck-$VERSION.linux.x86_64.tar.xz \
+            -O shellcheck.tar.xz
+          tar -xf shellcheck.tar.xz
+          mv shellcheck-$VERSION/shellcheck .
+          rm -rf shellcheck.tar.xz shellcheck-$VERSION
+
+      - name: Run Shellcheck
+        run: |
+          ./shellcheck --version
+          ./shellcheck -e SC2148 wt completions/*