ci: add check-spelling (#462)

This is a basic spell checker to make sure words are spelled correctly.

Signed-off-by: Xe Iaso <me@xeiaso.net>
Co-authored-by: Xe Iaso <me@xeiaso.net>

.github/actions/spelling/advice.md

@@ -0,0 +1,31 @@
+<!-- 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.