chore(flutter_bloc): add screenshots to pubspec.yaml (#3717)

Author: felangel

.github/workflows/flutter_bloc.yaml

@@ -60,6 +60,7 @@ jobs:
         run: |
           flutter packages get
           flutter pub global activate pana
+          sudo apt-get install webp
 
       - name: Verify Pub Score
         run: ../../tool/verify_pub_score.sh

packages/flutter_bloc/pubspec.yaml

@@ -18,3 +18,7 @@ dependencies:
 dev_dependencies:
   flutter_test:
     sdk: flutter
+
+screenshots:
+  - description: The flutter bloc package logo.
+    path: screenshots/logo.png