Implement snap-on-input for conhost

[lhecker]

Once COOKED_READ_DATA uses VT sequences to position the cursor,
nothing will snap the viewport back to the prompt line anymore.
The same problem already exists with WSL. Unfortunately, the inbox
conhost in build ~26100 has the same issue.

This PR implements a "snap on input" functionality similar to the one
in Windows Terminal by ignoring control keys and otherwise snapping
the viewport to the current cursor position. This is slightly different
to Windows Terminal however, because it should actually snap to the
virtual viewport position instead. The benefit of this difference is
that the diff is way small which hopefully makes backporting easier.

[lhecker]

I've implemented snapping to the virtual viewport on input here: dc0f798

I cannot quite decide what behavior is better though. If we snap to the virtual viewport then this will affect scroll-forward in conhost because it'll snap even if you're slightly scrolled down by a few lines.

[DHowett]

Notes from chat: i think we agreed on, "don't snap if the cursor is anywhere in view" (?)