Make sub-issues visible in GitHub project cards

[aminalhazwani]

Is your feature request related to a problem? Please describe.
It's not immediately clear that a issue has related sub-issues in the project view. For example this issue has 6 sub-issues but there is no information that makes it explicit.

Even in the detail view, inside in the project sidebar, this information is not immediately clear. You always have to open the issue in a new tab to see the related sub-issues.

Describe the solution you'd like
We could use a solution similar to the task progress count, where the number of completed/not completed tasks is immediately visible:

Additional context
We could use GitHub markup and styles to implement this feature with less effort, specifically:

Using the checkbox markup and CSS classes

<input type="checkbox" id="" class="task-list-item-checkbox">

And, the task progress count markup and classes:

<span class="issue-meta-section task-progress ">
  <svg class="octicon octicon-checklist" viewBox="0 0 16 16" version="1.1" width="16" height="16" aria-hidden="true">
    <path fill-rule="evenodd" d="M16 8.5l-6 6-3-3L8.5 10l1.5 1.5L14.5 7 16 8.5zM5.7 12.2l.8.8H2c-.55 0-1-.45-1-1V3c0-.55.45-1 1-1h7c.55 0 1 .45 1 1v6.5l-.8-.8c-.39-.39-1.03-.39-1.42 0L5.7 10.8a.996.996 0 0 0 0 1.41v-.01zM4 4h5V3H4v1zm0 2h5V5H4v1zm0 2h3V7H4v1zM3 9H2v1h1V9zm0-2H2v1h1V7zm0-2H2v1h1V5zm0-2H2v1h1V3z"></path>
  </svg>
  <span class="task-progress-counts">1 of 4</span>
  <span class="progress-bar v-align-middle"></span>
  <span class="progress" style="width: 25%"></span>
</span>