[vi-mode] Deleting backward to|until a character preserves the character under the cursor

[springcomp]

PR Summary

Fixes #1674.

In Vi mode, deleting to backward text should not remove the character under (at the right) of the cursor. This PR adjusts the boundary used by the DeleteBackwardToEndPoint to preserve the last (starting) character.

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
• 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 expecting documentation change.

Microsoft Reviewers: Open in CodeFlow

[ghost]

🎉 v2.2.0-beta2 has been released which incorporates this pull request. 🎉