[json] show number of children in JSON arrays

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> <!-- Describe the feature you'd like. --> This could be done either on hover or inline the way variable previews are shown while debugging. Screenshot what I'm referring to for hover with my mouse over the `[`: <img width="520" alt="Screenshot 2021-05-21 at 2 34 03 PM" src="https://user-images.githubusercontent.com/8371943/119189518-c410ad80-ba41-11eb-936e-4e6e0a1563a5.png">