[Accessibility experiment - Keyboard navigation of toolbox] Can't get past extensions menu in toolbox

[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. Pressing S will take you through the toolbox menus but when you reach Extensions the extensions menu fills the screen and gets stuck (further use of WASD do nothing). This also means it isn't possible to reach the Advanced menu in the Toolbox. At this point, if the user tries the backspace button, they will delete whatever code was highlighted on their workspace when they entered the toolbox.

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. Note that 'On start' is highlighted in the workspace
4. Press T to enter the toolbox menu
5. Press S and W to move up and down the toolbox options
6. Press S until you reach the Extensions menu
7. Then try S, D, W, A keys - none of them have an effect, you are trapped in this menu unless you use the mouse
8. Press Backspace
9. Now use the mouse to close the extensions menu and note that the 'On start' block has been deleted while the extension menu was on screen.

https://github.com/microsoft/pxt-microbit/assets/102724242/43f3280a-6e4c-4c32-bdae-6619f5f50117

Expected behavior A clear and concise description of what you expected to happen.