Keyboard nav allows creation of comments that can't be accessed by mouse

[microbit-matt-hillsdon]

You can't normally create a comment here but you can with the keyboard access to the context menu:

...but then you can't access it as a mouse user.

It can't be interacted with as a keyboard user either but that feels more of a limitation to be addressed, whereas the mouse issue is more awkward.

[rachel-fenichel]

Some related comment-creation tests in scratch:

And you actually can also put a comment on a single-field block in MakeCode right now, and then can't click on it because the click opens the field editor.

However, if you right-click anywhere on this block, including on the shadow blocks (0 and 1) the comment is applied to the parent block:

[rachel-fenichel]

There are two possible bugs here:

1. The way we're triggering the context menu on a shadow block uses that block instead of its parents for actions, including add comment. Normally the code in [Gesture.setTargetBlock](https://github.com/google/blockly/blob/develop/core/gesture.ts#L1042) directs actions to the parent block.
2. The precondition for the add comment action in the context menu needs to be stricter.