[Accessibility experiment - keyboard navigation of toolbox] Can't get out of toolbox sub menu

[microbit-lucy]

Describe the bug If you turn on the keyboard navigation 'accessibility' experiment, you can access the toolbox by pressing T. In the experiment the WASD keys should allow you to navigate this menu. 'D' correctly takes you into a sub-menu but press 'A' does not take you out (it does nothing) so the user gets trapped inside the sub-menu (although Esc will take you back to the workspace).

To Reproduce Steps to reproduce the behavior:

1. Go to https://makecode.microbit.org/#editor and turn on the experiment via Settings -> About -> Experiments
2. Turn on the Experiment by choosing: Settings -> Accessible blocks on
3. Press T to enter the toolbox
4. Press S and W to move up and down the toolbox options
5. Press D to go into a sub menu (e.g. for Basic)
6. Press A - you will see that nothing happens.

https://github.com/microsoft/pxt-microbit/assets/102724242/4c797429-b494-4a5f-94f7-11110140de26

Expected behavior Pressing A should take you out of the sub-menu.

Ideally, it should also be possible to reach the toolbox using Tab then Enter to open a sub-menu, then up/down or tab/shift+tab to navigate within the sub-menu and Esc to come out of the sub menu as well. This would ensure the toolbox menus work the same as the main navigation menus (e.g. Settings menu) for consistency.