feat: add gh action for vale check

irt https://github.com/blockstackpbc/devops/issues/527

considering this known Vale issue: https://github.com/errata-ai/vale-action/issues/27

.github/workflows/stylecheck-prs.yml

@@ -0,0 +1,25 @@
+name: Stylecheck
+on:
+  pull_request:
+    branches: [master]
+
+jobs:
+  stylecheck:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Get Changed Files
+        id: get_changed_files
+        uses: lots0logs/gh-action-get-changed-files@2.1.4
+        with:
+          token: ${{ github.token }}
+      - name: Run vale
+        uses: errata-ai/vale-action@v1.3.0
+        with:
+          styles: https://github.com/errata-ai/Google/releases/latest/download/Google.zip
+          config: https://raw.githubusercontent.com/blockstack/docs/master/.vale/vale.ini
+          files: "${{ steps.get_changed_files.outputs.all }}"
+          onlyAnnotateModifiedLines: true
+        env:
+          GITHUB_TOKEN: ${{ github.token }}