[3.13] gh-137450: macOS installer shell path management improvements (GH-137451) (#137452)

Separate the installer `Shell profile updater` postinstall script from the `Update Shell Profile.command` to enable more robust error handling.
(cherry picked from commit 781eb1a688e0575a38c79575493f3ff83208a33e)

Co-authored-by: Ned Deily <nad@python.org>