add bc-check actions workflow

Author: michael-rubel

.github/workflows/bc-check.yml

@@ -0,0 +1,26 @@
+name: bc-check
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - main
+
+jobs:
+  backwards-compatibility-check:
+    name: Backwards Compatibility Check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+      - name: "Install PHP"
+        uses: shivammathur/setup-php@v2
+        with:
+          php-version: "8.0"
+      - name: "Install dependencies"
+        run: "composer install"
+      - name: "Check for BC breaks"
+        run: "vendor/bin/roave-backward-compatibility-check --format=github-actions"