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 #3857.

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

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[totkeks]

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