Remove the duplication from project-switch-commands's config

Remove the duplication from project-switch-commands's config

Based on an older patch by Philip K (https://debbugs.gnu.org/41890#127).

* lisp/progmodes/project.el: (project-switch-commands): Change to
'defcustom', alter the value format, add :type.
(project-switch-use-entire-map): New option.
(project--keymap-prompt, project-switch-project):
Update accordingly, while keeping compatibility with user-defined
values in the previous format (for some transition period).

Co-authored-by: Philip K. <philipk@posteo.net>