Show parts of the scrollback on restore

[lhecker]

First, this makes use of PSEUDOCONSOLE_INHERIT_CURSOR to stop ConPTY from emitting a CSI 2 J on startup. Then, it uses Terminal::SetViewportPosition to fake-scroll the viewport down so that only 3 lines of scrollback are visible. It avoids printing actual newlines because if we later change the text buffer to actually track the written contents, we don't want those newlines to end up in the next buffer snapshot.

Closes #17274

Validation Steps Performed

• Restore cmd multiple times
• There's always exactly 3 lines visible ✅

[DHowett]

Does this approach cause the display to snap down to the bottom when you type for the first time?

[lhecker]

It will already be at the bottom when you open it:

If I scroll up and then type, it'll snap down to how it looks like in the screenshot.