Add an action to the `clear_terminal` function to scroll the screen contents into the scrollback buffer

Now if you want to map a key combination that will both scroll the
current screen contents into the scrollback buffer and clear the screen,
you can do something like:

map ctrl+l combine : clear_terminal scroll active : send_text normal,application \x0c

Fixes #1113

docs/changelog.rst

@@ -208,6 +208,9 @@ Changelog
 - Add an option :opt:`env` to set environment variables in child processes
   from kitty.conf
 
+- Add an action to the ``clear_terminal`` function to scroll the screen
+  contents into the scrollback buffer (:iss:`1113`)
+
 - Implement high precision scrolling with the trackpad on platforms such as
   macOS and Wayland that implement it. (:pull:`819`)