Fix 2 menu completion issues that happen when we need to scroll screen buffer

[daxian-dbw]

PR Summary

Fix #2928
Fix #2948

The details of the issues and root cause can be found at #2928 (comment) and #2948 (comment)

Major changes in this PR:

1. Handle moving the cursor to the start-drawing position appropriately, taking screen buffer scrolling in consideration.
2. Move duplicate code into the function MoveCursorToStartDrawingPosition and used it for both menu completion and dynamic help.
3. Initialize PreviousTop correctly before Top is changed, and get rid of bufferEndPoint.Y because we should compare Top with PreviousTop to determine if menu moved.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Make sure you've added one or more new tests
  - Cannot add tests because we cannot mimic buffer scrolling with the testing console, but I manually validated the fix
• Make sure you've tested these changes in terminals that PowerShell is commonly used in (i.e. conhost.exe, Windows Terminal, Visual Studio Code Integrated Terminal, etc.)
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow

[daxian-dbw]

@andschwa Can you please do a sanity check for me? Thanks!

[ghost]

🎉 v2.2.0-beta5 has been released which incorporates this pull request. 🎉