chore: github action to run prettier

Matches w3c/graphics-aria#10, including dependabot for the action.

.github/workflows/dependabot.yaml

@@ -0,0 +1,12 @@
+# Set update schedule for GitHub Actions
+
+version: 2
+updates:
+
+  - package-ecosystem: "github-actions"
+    directory: "/"
+    schedule:
+      interval: "weekly"
+    open-pull-requests-limit: 10
+    commit-message:
+      prefix: "chore: dependabot"

.github/workflows/prettier.yaml

@@ -0,0 +1,26 @@
+name: Prettier
+# minimally modified from https://github.com/creyD/prettier_action#example-2-using-the-only_changed-or-same_commit-option-on-pr
+
+on:
+  pull_request:
+    branches: 
+      - main
+
+jobs:
+  prettier:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v3
+        with:
+          # Make sure the actual branch is checked out when running on pull requests
+          ref: ${{ github.head_ref }}
+          # This is important to fetch the changes to the previous commit
+          fetch-depth: 0
+
+      - name: Prettify code
+        uses: creyD/prettier_action@v4.3
+        with:
+          prettier_options: --write *.html --print-width 200
+          commit_message: "chore: prettier"