Add workflow to block merges to main

With the migration of the feature plugin into the `woocommerce/woocommerce` repository, we're going to use this workflow to block content from being merged into `main`.

plugins/woocommerce-admin/.github/workflows/block-merge-to-main.yml

@@ -0,0 +1,10 @@
+name: 'Block Merge To main'
+on: [ pull_request ]
+
+jobs:
+  block_merge:
+    runs-on: ubuntu-latest
+    steps:
+    - name: Merge Blocked
+      if: github.event.pull_request.base.ref == 'main'
+      run: exit 1