feat: update editable region behaviour (#43537)
feat: update editable region behaviour (#43537)

* refactor: remove ambiguity about editable region

Since the editable region is implemented via decorations and defined in
challenge object, getEditableRegionFromRedux, makes the source obvious

* fix: make jaws follow the highlighted region

* fix: update the jaws on all content changes

* feat: make editable region 'absorb' text

As the user types, the editable region can move, expand and contract.

With this PR then if the user, say, presses backspace on the line after
the editable region, causing that line to move up, then the new contents
will expand/contract as if they had always been part of the region.
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