Add autoformatting of pixeebot PRs (#389)

Stole this script from another project that will automatically
auto-format pixeebot PRs.

Author: nahsra

.github/workflows/format_pixeebot_prs.yaml

@@ -0,0 +1,34 @@
+name: Format Pixeebot PRs
+
+on:
+  pull_request:
+    types: [opened, synchronize]
+
+jobs:
+  spotless-apply:
+    if: github.event.pull_request.user.login == 'pixeebot[bot]'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: write
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: "Setup JDK"
+        uses: actions/setup-java@v4
+        with:
+          distribution: 'temurin'
+          java-version: '17'
+
+      - name: 🐘Setup Gradle
+        uses: gradle/actions/setup-gradle@v3
+        with:
+          cache-read-only: true
+
+      - name: 🎨 Run spotless via Gradle
+        run: ./gradlew spotlessApply
+
+      - name: Commit and push changes
+        uses: stefanzweifel/git-auto-commit-action@v5
+        with:
+          commit_message: ":art: Apply formatting"