Add spelling check to GitHub workflow (#3623)

Besides updating cspell and handling spelling issues, the important
change is adding the spelling check to the GitHub workflow.

I'm not sure if it will bother us too much when people create PRs. But I
wanted to give it a try. Or do you have any other ideas on how we can
run the spelling check on a regular basis?

.github/workflows/spellcheck.yaml

@@ -0,0 +1,31 @@
+# This workflow will run a spellcheck on the codebase.
+# It runs a few days before each release. At 00:00 on day-of-month 27 in March, June, September, and December.
+
+name: Run Spellcheck
+
+on:
+  schedule:
+    - cron: "0 0 27 3,6,9,12 *"
+
+permissions:
+  contents: read
+
+jobs:
+  spellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+        with:
+          ref: develop
+      - name: Set up Node.js
+        uses: actions/setup-node@v4
+        with:
+          node-version: "22"
+          check-latest: true
+          cache: "npm"
+      - name: Install dependencies
+        run: |
+          npm run install-mm:dev
+      - name: Run Spellcheck
+        run: npm run test:spellcheck