Add spell checking GitHub Actions workflow

Follow up to rubocop/rubocop#10128.

This PR adds spell checking GitHub Actions workflow.

.github/workflows/spell_checking.yml

@@ -0,0 +1,33 @@
+name: Spell Checking
+
+on: [pull_request]
+
+jobs:
+  codespell:
+    name: Check spelling with codespell
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: [3.8]
+    steps:
+      - uses: actions/checkout@v2
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v2
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install codespell
+          if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
+      - name: Check spelling with codespell
+        run: codespell --ignore-words=codespell.txt || exit 1
+  misspell:
+    name: Check spelling with misspell
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Install
+        run: wget -O - -q https://git.io/misspell | sh -s -- -b .
+      - name: Misspell
+        run: ./misspell -error

codespell.txt

@@ -0,0 +1,3 @@
+revered
+rouge
+sting