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

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`.

Start by reproducing the shortcut behavior on the tip of main and compare it with Jupyter Notebook 6, using issue #7041 for context. Done means holding Shift while pressing Right Arrow selects from the current cell through the end of the section.

Written by the indexing model from the issue text.