gh-95511: IDLE - fix Shell context menu copy-with-prompts bug (GH-95512)

If one selects whole lines, as the sidebar makes easy, do not
add an extra line.  Only move the end of a selection to the
beginning of the next line when not already at the beginning
of a line.  (Also improve the surrounding code.)
(cherry picked from commit fc31a13dc1799b8d972c1f4ea49f27090aed7f48)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>