feat(CI): check for trailing whitespace

Run on pull request only.
Use a shell script to check for trailing whitespace in all files.
Fail if trailing whitespace is found.

.github/workflows/lint.yml

@@ -31,3 +31,12 @@ jobs:
           node-version: '12.x'
       - run: npm install -g markdownlint-cli@0.25.0
       - run: markdownlint '**/*.md'
+  trailing-whitespace:
+    name: 🌌 Trailing whitespace
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: 🧹 Check for trailing whitespace
+        run: |
+          cd test || exit
+          sh ./check-for-trailing-whitespace.sh || exit 1