Add `workflow_dispatch`

So that worfklows can be triggered.
https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#workflow_dispatch

.github/workflows/karma-tests.yml

@@ -8,6 +8,7 @@ on:
     branches: [ master ]
   pull_request:
     branches: [ master ]
+  workflow_dispatch
 
 jobs:
   build: