ci: spell check source via codespell

Integrate the codespell github action to spell check the source
code in CI.

.github/workflows/ci.yml

@@ -39,3 +39,12 @@ jobs:
       uses: ludeeus/action-shellcheck@0.5.0
       with:
         severity: warning
+
+  codespell:
+    name: "Spell check"
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: codespell-project/actions-codespell@master
+        with:
+          skip: ./.git,build