ci: Add spellcheck task

Based on slog-rs/slog#6b98d9e

Author: Techcable

.github/workflows/spellcheck.yml

@@ -0,0 +1,20 @@
+# Checks spelling with typos
+# This has very few false positives, only checking for known misspellings (similar to codespell).
+# This action is based on https://github.com/crate-ci/typos/blob/master/docs/github-action.md
+name: Check Spelling
+on: [push, pull_request]
+
+env:
+  CLICOLOR: 1
+
+permissions:
+  contents: read
+
+jobs:
+  typos:
+    name: Check spelling with typos
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - name: Run typos (pinned version)
+      uses: crate-ci/[email protected]

.typos.toml

@@ -0,0 +1,13 @@
+[default]
+extend-ignore-re = [
+    # support spellchecker directives (ideally this would be builtin)
+    "(?Rm)^.*(#|//)\\s*spellchecker:ignore$",
+    "(?s)(#|//)\\s*spellchecker:off.*?\\n\\s*(#|//)\\s*spellchecker:on",
+]
+
+[files]
+ignore-hidden = false
+extend-exclude = [
+    ".git", # needed because we set 'ignore-hidden'
+]
+