[folding] Add `unfold level [number]`

<!-- ⚠️⚠️ 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. --> Similar to the `fold level [number]` functionality, it would be very convenient to add a `unfold level [number]` functionality. The `fold level` assumes that the user is viewing the file when it is entirely unfolded. When viewing a large file I always start out with folding the entire file to get a clear overview of the file. In this scenario, a `unfold level` would be very useful. Currently, I use the `unfold` command and manually take the cursor to the next level and unfold again. Thank you!