Add check-spelling CI action

Add CI to catch spelling errors. See https://www.check-spelling.dev/ for information.

Initially this will only check the serde directory, but the intention is to expand its coverage as spelling errors in other directories are fixed.

Note that for this to work the action should be made a required check, otherwise when a typo is added forks from that commit will get complaints.

If a typo is intentional, the action will provide information about how to add it to expect.txt such that it will be accepted as an expected item (i.e. not a typo).

To skip a file/directory entirely, add a matching entry to excludes.txt.