Add check-spelling

.github/actions/spelling/README.md

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+# check-spelling/check-spelling configuration
+
+File | Purpose | Format | Info
+-|-|-|-
+[dictionary.txt](dictionary.txt) | Replacement dictionary (creating this file will override the default dictionary) | one word per line | [dictionary](https://github.com/check-spelling/check-spelling/wiki/Configuration#dictionary)
+[allow.txt](allow.txt) | Add words to the dictionary | one word per line (only letters and `'`s allowed) | [allow](https://github.com/check-spelling/check-spelling/wiki/Configuration#allow)
+[reject.txt](reject.txt) | Remove words from the dictionary (after allow) | grep pattern matching whole dictionary words | [reject](https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples%3A-reject)
+[excludes.txt](excludes.txt) | Files to ignore entirely | perl regular expression | [excludes](https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples%3A-excludes)
+[only.txt](only.txt) | Only check matching files (applied after excludes) | perl regular expression | [only](https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples%3A-only)
+[patterns.txt](patterns.txt) | Patterns to ignore from checked lines | perl regular expression (order matters, first match wins) | [patterns](https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples%3A-patterns)
+[candidate.patterns](candidate.patterns) | Patterns that might be worth adding to [patterns.txt](patterns.txt) | perl regular expression with optional comment block introductions (all matches will be suggested) | [candidates](https://github.com/check-spelling/check-spelling/wiki/Feature:-Suggest-patterns)
+[line_forbidden.patterns](line_forbidden.patterns) | Patterns to flag in checked lines | perl regular expression (order matters, first match wins) | [patterns](https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples%3A-patterns)
+[expect.txt](expect.txt) | Expected words that aren't in the dictionary | one word per line (sorted, alphabetically) | [expect](https://github.com/check-spelling/check-spelling/wiki/Configuration#expect)
+[advice.md](advice.md) | Supplement for GitHub comment when unrecognized words are found | GitHub Markdown | [advice](https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples%3A-advice)
+[block-delimiters.list](block-delimiters.list) | Define block begin/end markers to ignore lines of text | line with _literal string_ for **start** followed by line with _literal string_ for **end** | [block ignore](https://github.com/check-spelling/check-spelling/wiki/Feature%3A-Block-Ignore#status)
+
+Note: you can replace any of these files with a directory by the same name (minus the suffix)
+and then include multiple files inside that directory (with that suffix) to merge multiple files together.

.github/actions/spelling/advice.md

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+<!-- See https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples%3A-advice --> <!-- markdownlint-disable MD033 MD041 -->
+<details><summary>If the flagged items are :exploding_head: false positives</summary>
+
+If items relate to a ...
+* binary file (or some other file you wouldn't want to check at all).
+
+  Please add a file path to the `excludes.txt` file matching the containing file.
+
+  File paths are Perl 5 Regular Expressions - you can [test](
+https://www.regexplanet.com/advanced/perl/) yours before committing to verify it will match your files.
+
+  `^` refers to the file's path from the root of the repository, so `^README\.md$` would exclude [README.md](
+../tree/HEAD/README.md) (on whichever branch you're using).
+
+* well-formed pattern.
+
+  If you can write a [pattern](
+https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples:-patterns
+) that would match it,
+  try adding it to the `patterns.txt` file.
+
+  Patterns are Perl 5 Regular Expressions - you can [test](
+https://www.regexplanet.com/advanced/perl/) yours before committing to verify it will match your lines.
+
+  Note that patterns can't match multiline strings.
+
+</details>
+
+<!-- adoption information-->
+:steam_locomotive: If you're seeing this message and your PR is from a branch that doesn't have check-spelling,
+please merge to your PR's base branch to get the version configured for your repository.

.github/actions/spelling/allow.txt

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+github
+https
+ssh
+ubuntu
+workarounds

.github/actions/spelling/block-delimiters.list

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+# Public Keys
+-----BEGIN PUBLIC KEY-----
+-----END PUBLIC KEY-----
+
+# RSA Private Key
+-----BEGIN RSA PRIVATE KEY-----
+-----END RSA PRIVATE KEY-----
+
+# GPG Public Key
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+-----END PGP PUBLIC KEY BLOCK-----
+
+# Certificates
+-----BEGIN CERTIFICATE-----
+-----END CERTIFICATE-----