Automatically format YAML, CSS, and JS files (#1272)

Initially, I also formatted the templates in `theme/`, but this triggers
https://github.com/prettier/prettier/issues/11834. So I exclude them for
now.

---------

Co-authored-by: Ming-Ying Chung <mych@chromium.org>

.github/workflows/labeler.yml

@@ -1,6 +1,6 @@
 name: "Pull Request Labeler"
 on:
-- pull_request_target
+  - pull_request_target
 
 jobs:
   triage:
@@ -9,6 +9,6 @@ jobs:
       pull-requests: write
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/labeler@v4
-      with:
-        sync-labels: true
+      - uses: actions/labeler@v4
+        with:
+          sync-labels: true