SHIFT+RIGHT shortcut does not select from the current cell to the end of the section

[JasonWeill]

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This is a spinoff from #7041, as noted by @jph00, as a regression from Jupyter Notebook 6 to 7:

Some of the shortcuts from the Collapsible Headings extension have been added - thank you! However, the ability to hold shift whilst pressing right-arrow has been lost -- this ought to select from the current cell to the end of the section

Issue 7041 was opened on 2023-09-12, but I can still reproduce this today using the tip of main.