Add a Spellcheck GitHub Action and config

.github/workflows/spellcheck.yml

@@ -0,0 +1,17 @@
+name: 'Check spelling'
+on: # rebuild any PRs and main branch changes
+  push:
+    branches-ignore:
+      - "dependabot/**"
+  pull_request:
+
+jobs:
+  spellcheck:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - uses: streetsidesoftware/cspell-action@3bc52c39528d0214a947ff7f8b5c23ad3273435b
+        with:
+          inline: warning
+          strict: false
+          incremental_files_only: true