Keyboard navigation for block comments

[microbit-matt-hillsdon]

Similar to #625 and the related blockly change, it would be good to support navigation for block comments.

The most critical issue is that you can get stuck with the comment as the only keyboard reachable position in the workspace once you tab to a comment. A fix for that that left us short of full navigation would still be valuable but full navigation might be the most viable route.

MakeCode actually has a custom implementation more like workspace comments so perhaps work done for workspace nav can help us fix this in MakeCode itself independently of support in the regular block comment? Any advice welcome.

Some thoughts on comment navigation more broadly in this doc..

[microbit-matt-hillsdon]

Discussed with the MakeCode team. We're going to support Escape to collapse a block comment (in MakeCode's custom comment bubble) as an interim approach to getting stuck, then consider this further after the release.