[James Holderness (j4james)]

The main purpose of this PR was to merge the ITerminalApi::LineFeed
implementations into a shared method in AdaptDispatch, and avoid the
VT code path depending on the AdjustCursorPosition function (which
could then be massively simplified). This refactoring also fixes some
bugs that existed in the original LineFeed implementations.

References and Relevant Issues

This helps to close the gap between the Conhost and Terminal (#13408).
This improves some of the scrollbar mark bugs mentioned in #11000.

Detailed Description of the Pull Request / Additional comments

I had initially hoped the line feed functionality could be implemented
entirely within AdaptDispatch, but there is still some Conhost and
Terminal-specific behavior that needs to be triggered when we reach the
bottom of the buffer, and the row coordinates are cycled.

In Conhost we need to trigger an accessibility scroll event, and in
Windows Terminal we need to update selection and marker offsets, reset
pattern intervals, and preserve the user's scroll offset. This is now
handled by a new NotifyBufferRotation method in ITerminalApi.

But this made me realise that the _EraseAll method should have been
doing the same thing when it reached the bottom of the buffer. So I've
added a call to the new NotifyBufferRotation API from there as well.

And in the case of Windows Terminal, the scroll offset preservation was
something that was also needed for a regular viewport pan. So I've put
that in a separate _PreserveUserScrollOffset method which is called
from the SetViewportPosition handler as well.

Validation Steps Performed

Because of the API changes, there were a number of unit tests that
needed to be updated:

• Some of the ScreenBufferTests were accessing margin state in the
  SCREEN_INFORMATION class which doesn't exist anymore, so I had to add
  a little helper function which now manually detects the active margins.

• Some of the AdapterTest tests were dependent on APIs that no longer
  exist, so they needed to be rewritten so they now check the resulting
  state rather than expecting a mock API call.

• The ScrollWithMargins test in ConptyRoundtripTests was testing
  functionality that didn't previously work correctly (issue Scrolling within regions that include the top of the screen doesn't push lines into scrollback #3673). Now
  that it's been fixed, that test needed to be updated accordingly.

Other than getting the unit tests working, I've manually verified that
issue #3673 is now fixed. And I've also checked that the scroll markers,
selections, and user scroll offset are all being updated correctly, both
with a regular viewport pan, as well as when overrunning the buffer.

Closes #3673

[Dustin L. Howett (DHowett)]

I didn't notice in the PR description that margins also moved into AdaptDispatch! Very clever, I love the new better encapsulation.

[Dustin L. Howett (DHowett)]

qq: who turns it back on?

[James Holderness (j4james)]

There's a call to _ApplyCursorMovementFlags at the end of the method which turns the cursor back on.

[Dustin L. Howett (DHowett)]

It all makes sense to me! I'd love to get Mike Griese (@zadjii-msft)'s or Leonard Hecker (@lhecker)'s hands on the ball as well.

Thanks for being so patient with us, James Holderness (@j4james)!

[Mike Griese (zadjii-msft)]

Thank you for remedying the abomination that was AdjustCursorPosition - that got wildly out of hand during windows 10 and I absolutely hated every minute of it. This seems a lot easier to parse.