add spellcheck to CI

[jmayclin]

Problem:

We spend time correcting spelling mistakes in CI.

Solution:

Let's just add a spellchecker to CI.

Requirements / Acceptance Criteria:

Most spelling mistakes should be caught by automated tooling.

Out of scope:

We don't need a perfect solution, just a better solution.

[camshaft]

In s2n-quic we use typos, which has worked quite well. You can see the workflow setup here: https://github.com/aws/s2n-quic/blob/df48d3cc378297f1e474abfe117483c22fb3b922/.github/workflows/ci.yml#L713-L735