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>
(cherry picked from commit 9fabcfbe68)

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

Lib/idlelib/idle_test/example_noext

@@ -0,0 +1,4 @@
+#!usr/bin/env python
+
+def example_function(some_argument):
+    pass

Lib/idlelib/idle_test/example_stub.pyi

@@ -0,0 +1,2 @@
+class Example:
+    def method(self, argument1: str, argument2: list[int]) -> None: ...

Lib/idlelib/idle_test/test_iomenu.py

@@ -1,6 +1,6 @@
 "Test , coverage 17%."
 
-from idlelib import iomenu
+from idlelib import iomenu, util
 import unittest
 from test.support import requires
 from tkinter import Tk
@@ -45,5 +45,27 @@ def test_fixnewlines_end(self):
         eq(fix(), 'a'+io.eol_convention)
 
 
+def _extension_in_filetypes(extension):
+    return any(
+        f'*{extension}' in filetype_tuple[1]
+        for filetype_tuple in iomenu.IOBinding.filetypes
+    )
+
+
+class FiletypesTest(unittest.TestCase):
+    def test_python_source_files(self):
+        for extension in util.py_extensions:
+            with self.subTest(extension=extension):
+                self.assertTrue(
+                    _extension_in_filetypes(extension)
+                )
+
+    def test_text_files(self):
+        self.assertTrue(_extension_in_filetypes('.txt'))
+
+    def test_all_files(self):
+        self.assertTrue(_extension_in_filetypes(''))
+
+
 if __name__ == '__main__':
     unittest.main(verbosity=2)

Lib/idlelib/idle_test/test_util.py

@@ -0,0 +1,14 @@
+"""Test util, coverage 100%"""
+
+import unittest
+from idlelib import util
+
+
+class UtilTest(unittest.TestCase):
+    def test_extensions(self):
+        for extension in {'.pyi', '.py', '.pyw'}:
+            self.assertIn(extension, util.py_extensions)
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)