editorial: automatic formatting for all but aria spec

Adds:
- .prettierignore: ignore ARIA spec index.html
- .github/workflows/dependabot.yaml: check for updates
- .github/workflows/prettier.yaml: run prettier on */index.html
  (except what is specified in .prettierignore)

.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 +1,27 @@
+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
+      - monorepo*
+
+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/[email protected]
+        with:
+          prettier_options: --write **/index.html --print-width 200
+          commit_message: "chore: prettier"

.prettierignore

@@ -0,0 +1 @@
+/index.html