Enable tab completion for variable assignment that is enum

[SteveL-MSFT]

PR Summary

Add code to CompletionAnalysis class to detect if completion is for variable assignment and the variable is an enum. Then use helper GetResultForEnum() to return appropriate results matching string to complete. Enum strings are always quoted and will match initial single or double quote. Needed to have similar code in 3 places to handle:

$ErrorActionPreference = <tab>
$ErrorActionPreference = '<tab> # in this case, it's a string completion
$ErrorActionPreference=<tab>

Although it's not in this gif, the results are sorted by enum name alphabetically.

PR Context

Fix #10631

PR Checklist

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    • Impact on PowerShell Editor Services which is used in the PowerShell extension for VSCode (which runs in a different PS Host).
    • Impact on Completions (both in the console and in editors) - one of PowerShell's most powerful features.
    • Impact on PSScriptAnalyzer (which provides linting & formatting in the editor extensions).
    • Impact on EditorSyntax (which provides syntax highlighting with in VSCode, GitHub, and many other editors).

[SteveL-MSFT]

@PoshChan please retry windows

[PoshChan]

@SteveL-MSFT, successfully started retry of PowerShell-CI-Windows

[rjmholt]

LGTM

[ghost]

🎉v7.0.0-preview.5 has been released which incorporates this pull request.:tada:

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