title: Pull Requests - Ensure linked tasks are closed before allowing PR completion
body: <p>We wish to create a workflow where pull requests are only allowed to be completed when all the related tasks are in "done" state to avoid incongruencies. </p>
  
  <p>We know how to block pull requests from being completed unless there is at least one task assigned to it, but we didn't find a way to force Azure DevOps to check the tasks states before allowing said completion.</p>
  
  <p>Right now completing a pull requests sets the related tasks to "done", but we need it to check the state is already "done" before completion, as we want our developers to confirm task completion manually.</p>