Fix handling of surrogate pairs in length calculation

[totkeks]

PR Summary

Adds new conditions to handle surrogate pairs when calculating the buffer length of a string.

Fixes https://github.com/PowerShell/PSReadLine/issues/3857.

PR Checklist

• [X] PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• [X] Summarized changes
• [ ] Make sure you've added one or more new tests
• [X] 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
  • [X] Not Applicable

Microsoft Reviewers: Open in CodeFlow

[totkeks]

@microsoft-github-policy-service agree

[springcomp]

I think this pull request needs a unit-test to demonstrate the requirements.