turtle.pyi: Add functions copied from _Screen (#5104)

The turtle module re-exports methods of its classes as top-level functions.
Typeshed includes most of these, but `exitonclick` and others from the
`_Screen` class were missing.
Add them.

Co-authored-by: Petr Viktorin <encukou@gmail.com>

stdlib/turtle.pyi

@@ -295,8 +295,6 @@ class _Screen(TurtleScreen):
     def bye(self) -> None: ...
     def exitonclick(self) -> None: ...
 
-def Screen() -> _Screen: ...
-
 class Turtle(RawTurtle):
     def __init__(self, shape: str = ..., undobuffersize: int = ..., visible: bool = ...) -> None: ...
 
@@ -381,6 +379,14 @@ if sys.version_info >= (3,):
     def onkeypress(fun: Callable[[], Any], key: Optional[str] = ...) -> None: ...
     onkeyrelease = onkey
 
+# Functions copied from _Screen:
+
+def setup(width: float = ..., height: float = ..., startx: Optional[int] = ..., starty: Optional[int] = ...) -> None: ...
+def title(titlestring: str) -> None: ...
+def bye() -> None: ...
+def exitonclick() -> None: ...
+def Screen() -> _Screen: ...
+
 # Functions copied from TNavigator:
 
 def degrees(fullcircle: float = ...) -> None: ...