Add a TabGroups.move() method

<!-- ⚠️⚠️ 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. --> The demo below uses the previous `TabGroups.move()` method to make dragging tabs as `TreeItems` quite easy to do. The `move` method had the advantage over other commands as being able to operate on any `Tab` whether active or not. Without such ability using commands like `moveActiveEditor` requires that each tab one wishes to move be "activated", then moved and the previously active tab (if not moved) be "re-activated". I am investigating workarounds, see related https://github.com/microsoft/vscode/issues/148084 but a key command, `closeEditorAtIndex`, is not available and would require closing and opening an already opened tab just to move it to another group. I know there is some inherent complexity in implementing a `move` command that works for tabs as well as tabGroups, perhaps a `moveTab()` command can be implemented easier? In the demo I just loop through any moved group to apply the previous `move()` to each tab.