Toggle Delete Selection mode.
With a prefix argument ARG, enable Delete Selection mode if ARG
is positive, and disable it otherwise. If called from Lisp,
enable the mode if ARG is omitted or nil.
When Delete Selection mode is enabled, Transient Mark mode is also
enabled and typed text replaces the selection if the selection is
active. Otherwise, typed text is just inserted at point regardless of
any selection.
(fn &optional ARG)