Use FocusManager for LineCursor #8940
Description
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Problem
LineCursor needs to rely on FocusManager rather than selection in order to tie together focus management and keyboard navigation (which is a critical requirement for the complete end-to-end accessibility experience).
Request
This essentially requires that LineCursor use the current focused node by default and fall back to selection if that fails (though we will eventually be able to remove the selection bit entirely).
Alternatives considered
No response
Additional context
Note that this is essentially the tracking issue for Core changes to make RaspberryPiFoundation/blockly-keyboard-experimentation#142 possible. There are also prerequisite changes (such as critical components from #8771) and follow-up changes (specifically in the keyboard navigation plugin) in order for these changes to fully work.