Update 'CreateCharInfoBuffer' to support continuous 'NextLineToken'

[daxian-dbw]

PR Summary

Update 'CreateCharInfoBuffer' to support continuous NextLineToken.
The first 'NextLineToken' pads the current line to the end, and the following NextLineToken's will be interpreted as requests for empty lines.

This avoids the empty line strings created in many places in our tests, and makes the AssertScreenIs works more intuitively.

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 Applicable
  • OR
  • Documentation needed at PowerShell-Docs
    • Doc Issue filed:

Microsoft Reviewers: Open in CodeFlow

[ghost]

🎉 v2.2.0-beta4 has been released which incorporates this pull request. 🎉