github/workflows: Move codespell to a GitHub workflow, version it.

Similar to ruff.yaml, it's simpler to run the codespell command directly
from a workflow file.  And developers can run codespell directly from the
command line without the need for options, or just use pre-commit.

This commit also applies a specific version to codespell, same as
pre-commit (introduced in a166d805f4).

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

.github/workflows/codespell.yml

@@ -0,0 +1,12 @@
+name: Check spelling with codespell
+
+on: [push, pull_request]
+
+jobs:
+  codespell:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - run: pip install --user codespell==2.2.6 tomli
+    - run: codespell
+