Spell check

[jsoref]

This adds the check-spelling action. For more information, see check-spelling.dev.

It should have sufficient configurability & documentation to be used by most projects including this one. If you need features, I'm happy to work on them (or take PRs).

General workflows

1. If you have a fork w/ actions enabled, pushing to it will result in a comment identifying unrecognized words.
2. Once you make a PR, with suppress_push_for_open_pull_request, pushing will no longer trigger a comment on commits, instead relying on the destination PR's pull_request_target event to produce a comment.
3. With experimental_apply_changes_via_bot, you can quote the @check-spelling-bot line as a comment to ask the bot to apply the changes back to the PR branch.
4. GitHub will annotate the first 10 unrecognized items in the PR diff view. For additional items, the action log should list them.

Configuration

1. To exclude files, you can adjust the excludes.txt file.
2. If you only want to check certain paths, you can use only.txt.
3. You can add items to the dictionary using allow.txt, or by adding URLs to extra_dictionaries.
4. You can subtract patterns from the combined dictionary using reject.txt.
5. To mask patterns within a line, use patterns.txt.
6. Tokens which you currently tolerate but haven't added to a dictionary live in expect.txt -- if they disappear, the action will suggest removing them.