add post merge labeler (flutter#3532)

Author: bparrishMines

.github/post_merge_labeler.yml

@@ -0,0 +1,2 @@
+'needs-publishing':
+  - packages/*/**

.github/workflows/pull_request_label.yml

@@ -1,20 +1,31 @@
 # This workflow applies labels to pull requests based on the
 # paths that are modified in the pull request.
 #
-# Edit `.github/labeler.yml` to configure labels.
+# Edit `.github/labeler.yml` and `.github/post_merge_labeler.yml`
+# to configure labels.
 #
 # For more information, see: https://github.com/actions/labeler
 
 name: Pull Request Labeler
 
 on:
-  - pull_request_target
+  pull_request_target:
+    types: [opened, synchronize, reopened, closed]
 
 jobs:
   label:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/labeler@main
-      with:
-        repo-token: "${{ secrets.GITHUB_TOKEN }}"
-        sync-labels: true
+      - uses: actions/labeler@v3
+        with:
+          repo-token: "${{ secrets.GITHUB_TOKEN }}"
+          sync-labels: true
+
+  post_merge_label:
+    if: github.event.action == 'closed' && github.event.pull_request.merged == true
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/labeler@v3
+        with:
+          repo-token: "${{ secrets.GITHUB_TOKEN }}"
+          configuration-path: .github/post_merge_labeler.yml