Add utf8_enabled attribute to IMAP4 class (#15006)

stdlib/imaplib.pyi

@@ -26,6 +26,7 @@ class IMAP4:
     class error(Exception): ...
     class abort(error): ...
     class readonly(abort): ...
+    utf8_enabled: bool
     mustquote: Pattern[str]
     debug: int
     state: str