Accessibility test for jupyterLite

[smythp]

@szweibel I'll put any findings in this issue just to surface the work).

[smythp]

OK, so I tested this. Here are my notes:

In the JupyterLab version (there is also a retrolab version not tested), the menu for opening notebooks, console, etc. is broadly accessible with minor issues related to labeling

Once you access a notebook, there are some issues:

• Moving among cells doesn't have screen reader feedback, it says things like "blank" or gives no output
• It should have more and better alerts when running cells, but you do hear the output of the cell when you run it
• It seems that the toolbar works and is labeled, which is cool
• Landmarks have been implemented so you can move between toolbar, the main notebook view, and the files, which is also good

I only tested basic functionality. My initial asssessment is that if a few things were changed, the project would meet some minimum threshold for use with screen readers.