bpo-45447: Add syntax highlighting for `.pyi` files in IDLE (GH-28950)

bpo-45447: Add syntax highlighting for `.pyi` files in IDLE (GH-28950)

Also add .pyi to the python extensions in the "File-open" and "File-save" dialogues.
Add util.py to contain objects that are used in multiple idlelib modules
and have no dependencies on any of them.

Co-authored-by: E-Paine <63801254+E-Paine@users.noreply.github.com>
Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit 50cf4991c49e19f917305dd7b9c71085c11edddb)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>
(cherry picked from commit 9fabcfbe68ff81ef5f17f86a93daf9cce9d83876)

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