ci: add check-spelling GitHub Action

.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) |
+| [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)     |
+
+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 you see a bunch of garbage</summary>
+
+If it relates to a ...
+<details><summary>well-formed pattern</summary>
+
+See if there's a [pattern](https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples:-patterns) that would match it.
+
+If not, try writing one and 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>
+<details><summary>binary-ish string</summary>
+
+Please add a file path to the `excludes.txt` file instead of just accepting the garbage.
+
+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).
+</details>
+
+</details>

.github/actions/spelling/excludes.txt

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+# See https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples:-excludes
+(?:^|/)(?i)COPYRIGHT
+(?:^|/)(?i)LICEN[CS]E
+(?:^|/)package(?:-lock|)\.json$
+(?:^|/)vendor/
+ignore$
+\.avi$
+\.ico$
+\.jpe?g$
+\.lock$
+\.map$
+\.min\.
+\.mod$
+\.mp[34]$
+\.png$
+\.wav$
+^\.github/

.github/actions/spelling/expect.txt

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+ohmyzsh
+oh-my-zsh

.github/actions/spelling/patterns.txt

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+# See https://github.com/check-spelling/check-spelling/wiki/Configuration-Examples:-patterns
+
+# ignore long runs of a single character:
+\b([A-Za-z])\g{-1}{3,}\b

.github/actions/spelling/reject.txt

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+^attache$
+occurence
+Sorce
+^[Ss]pae
+^untill
+^wether

.github/workflows/spelling.yml

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+name: Spell checking
+on:
+  pull_request_target:
+  push:
+  issue_comment:
+    types: [created]
+
+jobs:
+  spelling:
+    name: Spell checking
+    runs-on: ubuntu-latest
+    steps:
+    - name: checkout-merge
+      if: "contains(github.event_name, 'pull_request')"
+      uses: actions/checkout@v2
+      with:
+        ref: refs/pull/${{github.event.pull_request.number}}/merge
+    - name: checkout
+      if: ${{ github.event_name == 'push' ||
+        (
+          contains(github.event.comment.body, '@check-spelling-bot apply')
+        ) }}
+      uses: actions/checkout@v2
+    - uses: check-spelling/check-spelling@main
+      id: spelling
+      if: ${{ github.event_name != 'issue_comment' ||
+        (
+          contains(github.event.comment.body, '@check-spelling-bot apply')
+        ) }}
+      with:
+        experimental_apply_changes_via_bot: 1