Add an overtype feature to the insert mode (sticky replace)

[ettoh]

Inspired by #11265 and #14120, I implemented a simple version of an overtype (sticky replace) mode. Instead of introducing a new mode, it uses the standard insert mode under the hood. This allows a single undo for all changes made while in the mode - which was a problem with other solutions so far.

By default, the mode can be entered with ctrl-r.

Implements #5843.

overtype_demo.mp4

[xpe]

Thanks for making this PR! I suggest just overtype_mode instead of enter_overtype_mode based on many examples from this table of static commands:

name	description	default keybinds
normal_mode	Enter normal mode	normal: <esc>, select: v, insert: <esc>
select_mode	Enter selection extend mode	normal: v
exit_select_mode	Exit selection mode	select: <esc>
insert_mode	Insert before selection	normal: i, select: i
append_mode	Append after selection	normal: a, select: a

[ettoh]

Thanks for making this PR! I suggest just overtype_mode instead of enter_overtype_mode based on many examples from this table of static commands:

@xpe Fair point, done!