Refine conditions for npm-package workflow trigger

Updated the workflow trigger logic to include merged pull requests targeting 'main', in addition to pushes to 'main' or branches starting with 'NP-'. This ensures proper handling of release branches

NP-732

.github/workflows/package.yml

@@ -47,7 +47,8 @@ jobs:
 
   npm-package:
     needs: [ conan-package ]
-    if: ${{ github.event_name == 'push' && (github.ref_name == 'main' || startsWith(github.ref_name, 'NP-') || startsWith(github.ref_name, '5.')) }}  # FIXME: have a more generic way to determine release branches
+    # Run the job for: 1) push to 'main' or branches starting with 'NP-', or 2) merged pull requests targeting 'main'.
+    if: ${{(github.event_name == 'push' && (github.ref_name == 'main' || startsWith(github.ref_name, 'NP-'))) || (github.event_name == 'pull_request' && github.event.action == 'closed' && github.base_ref == 'main' && github.merge_commit_sha)}}
     uses: ultimaker/cura-workflows/.github/workflows/npm-package.yml@main
     with:
       package_version_full: ${{ needs.conan-package.outputs.package_version_full }}