UseUsingScopeModifierInNewRunspaces: Fix ArgumentException when the same variable name is used in 2 different sessions.

[bergmeister]

PR Summary

Fixes #1492 by checking if the variable name was already added. Also simplifying the code to be a HashSet<string> instead of a Dictionary<string,VariableExpressionAst> since the actual variable seems to be never used. Unfortunately I have to drop the IReadOnlyCollection for HashSet since we target .Net 4.5.2 and only starting with .Net 4.6, HashSet is implementing IReadOnlyCollection, see here.
cc @Jawz84

PR Checklist

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• Summarized changes
• Change is not breaking
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[scott1138]

@bergmeister @rjmholt do you know when this will be added to a release?

[bergmeister]

@scott1138 We hope to release this next month