Add a clear-screen bind function to clear the screen (#10044)

This can be bound like `bind \cl clear-screen`, and is, by default

In contrast to the current way it doesn't need the external `clear`
command that was always awkward.

Also it will clear the screen and first draw the old prompt to remove
flicker.
Then it will immediately trigger a repaint, so the prompt will be overwritten.

doc_src/cmds/bind.rst

@@ -142,6 +142,9 @@ The following special input functions are available:
 ``capitalize-word``
     make the current word begin with a capital letter
 
+``clear-screen``
+    clears the screen and redraws the prompt. if the terminal doesn't support clearing the screen it is the same as ``repaint``.
+
 ``complete``
     guess the remainder of the current token