Fix tab completion for unlocalized about* topics

[MartinGC94]

Fixes tab completion for help topics on systems without localized help by adding en-US as a fallback.
Get-Help About<Tab>

PR Context

Get-Help will automatically fall back to en-US help topics so the completion for it should do the same.

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[iSazonov]

For discussion:

Suggested change

	Path.Combine(userHelpDir, currentCulture),
	Path.Combine(userHelpDir, "en-US"),
	Path.Combine(appHelpDir, currentCulture),
	Path.Combine(appHelpDir, "en-US")
	Path.Combine(userHelpDir, currentCulture),
	Path.Combine(appHelpDir, currentCulture),
	Path.Combine(userHelpDir, "en-US"),
	Path.Combine(appHelpDir, "en-US")

Maybe users want to get topics in their language first?

[ghost]

This pull request has been automatically marked as Review Needed because it has been there has not been any activity for 7 days.
Maintainer, please provide feedback and/or mark it as Waiting on Author

[adityapatwardhan]

@MartinGC94 Thank you for your contribution!

[ghost]

🎉v7.2.0-preview.6 has been released which incorporates this pull request.:tada:

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