Add spell checking to CI

Based on https://github.com/rubocop/rubocop/blob/master/.github/workflows/spell_checking.yml.

.github/workflows/spell_checking.yml

@@ -0,0 +1,30 @@
+name: Spell Checking
+
+on: [pull_request]
+
+permissions:  # added using https://github.com/step-security/secure-workflows
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+jobs:
+  codespell:
+    name: Check spelling of all files with codespell
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: codespell-project/actions-codespell@v2
+        with:
+          check_filenames: true
+          check_hidden: true
+  misspell:
+    name: Check spelling of all files in commit with misspell
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Install
+        run: wget -O - -q https://raw.githubusercontent.com/client9/misspell/master/install-misspell.sh | sh -s -- -b .
+      - name: Misspell
+        run: git ls-files --empty-directory | xargs ./misspell -error

spec/rubocop/cop/thread_safety/mutable_class_instance_variable_spec.rb

@@ -131,7 +131,7 @@ def some_method
         it_behaves_like 'mutable objects', '%w(a b c)'
         it_behaves_like 'mutable objects', '{ a: 1, b: 2 }'
         it_behaves_like 'mutable objects', "'str'"
-        it_behaves_like 'mutable objects', %("\#{30 + 12}nd")
+        it_behaves_like 'mutable objects', %("\#{30 + 12}ok")
 
         it_behaves_like 'immutable objects', '1'
         it_behaves_like 'immutable objects', '2.1'
@@ -391,7 +391,7 @@ def some_method
         it_behaves_like 'mutable objects', '%w(a b c)'
         it_behaves_like 'mutable objects', '{ a: 1, b: 2 }'
         it_behaves_like 'mutable objects', "'str'"
-        it_behaves_like 'mutable objects', %("\#{30 + 12}nd")
+        it_behaves_like 'mutable objects', %("\#{30 + 12}ok")
         it_behaves_like 'mutable objects', 'Something.new'
 
         it_behaves_like 'immutable objects', '1'