Add proposal validation workflow to pull_request workflow for testing

Author: dempseyatgithub

.github/workflows/pull_request.yml

@@ -96,3 +96,10 @@ jobs:
     with:
       api_breakage_check_enabled: false
       license_header_check_project_name: "Swift.org"
+
+  proposal_validation:
+      name: Proposal Validation
+      uses: ./.github/workflows/proposal_validation.yml
+      with:
+        project: "swift"
+        workflow_test_enabled: true