[3.9] bpo-32631: IDLE: Enable zzdummy example extension module (GH-14491)

Make menu items work with formatter, add docstrings, add 100% tests.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit e40e2a2cc9)

Co-authored-by: Cheryl Sabella <cheryl.sabella@gmail.com>

Lib/idlelib/configdialog.py

@@ -2316,7 +2316,15 @@ display when Code Context is turned on for an editor window.
 
 Shell Preferences: Auto-Squeeze Min. Lines is the minimum number of lines
 of output to automatically "squeeze".
-'''
+''',
+    'Extensions': '''
+ZzDummy: This extension is provided as an example for how to create and
+use an extension.  Enable indicates whether the extension is active or
+not; likewise enable_editor and enable_shell indicate which windows it
+will be active on.  For this extension, z-text is the text that will be
+inserted at or removed from the beginning of the lines of selected text,
+or the current line if no selection.
+''',
 }