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 50cf4991c4)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

Lib/idlelib/iomenu.py

@@ -11,6 +11,9 @@
 
 import idlelib
 from idlelib.config import idleConf
+from idlelib.util import py_extensions
+
+py_extensions = ' '.join("*"+ext for ext in py_extensions)
 
 encoding = 'utf-8'
 if sys.platform == 'win32':
@@ -348,7 +351,7 @@ def print_window(self, event):
     savedialog = None
 
     filetypes = (
-        ("Python files", "*.py *.pyw", "TEXT"),
+        ("Python files", py_extensions, "TEXT"),
         ("Text files", "*.txt", "TEXT"),
         ("All files", "*"),
         )