add api-breakage CI check (#15)

Author: sliemeobn

.github/workflows/api-breakage.yml

@@ -0,0 +1,22 @@
+name: API breaking changes
+
+on:
+  pull_request:
+
+jobs:
+  linux:
+    runs-on: ubuntu-latest
+    timeout-minutes: 15
+    container:
+      image: swift:6.0
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+      # https://github.com/actions/checkout/issues/766
+      - name: Mark the workspace as safe
+        run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
+      - name: API breaking changes
+        run: |
+          swift package diagnose-api-breaking-changes origin/${GITHUB_BASE_REF}