Prevent braces from being removed when completing variables

[MartinGC94]

PR Summary

Adds additional check for adding braces to the variable completion result if the user input includes braces. This fixes variable completion inside strings like: "Hello${psversiont<Tab>}World".
Also makes Null-conditional operator usage with variables slightly more convenient because tab completion won't remove the necessary braces from the variable name.

PR Context

PR Checklist

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  • Experimental feature(s) needed
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  • Not Applicable
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  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
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• Tooling
  • I have considered the user experience from a tooling perspective and don't believe tooling will be impacted.
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  • I have considered the user experience from a tooling perspective and opened an issue in the relevant tool repository. This may include:
    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode
      (which runs in a different PS Host).
      • Issue filed:
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
      • Issue filed:
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
      • Issue filed:
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).
      • Issue filed:

[daxian-dbw]

LGTM

[ghost]

🎉v7.3.0-preview.7 has been released which incorporates this pull request.:tada:

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