Fixed erroneous PSUseDeclaredVarsMoreThanAssignments for some globals variables

[John-Leitch]

PR Summary

This PR fixes the following longstanding issue: https://github.com/PowerShell/PSScriptAnalyzer/issues/1300 (Global variables are erroneously reported as "never used")

The issue exists because Helper.IsVariableGlobalOrEnvironment uses an Ast to key into VariableAnalysisDictionary, calling VariableAnalysis.IsGlobalOrEnvironment on the value returned to determine whether or not the variable is a global. However, in some cases, the key does not exist in the dictionary, resulting in the method returning false. Fortunately, VariableAnalysis.IsGlobalOrEnvironment depends on no instance members, instead relying solely on the properties of VariableExpressionAst, which is passed in. As such, VariableAnalysis.IsGlobalOrEnvironment can be made static with no consequence, allowing Helper.IsVariableGlobalOrEnvironment to call the method without the dictionary operations.

Also of note, the current implementation is somewhat redundant, with UseDeclaredVarsMoreThanAssignments.AnalyzeScriptBlockAst checking VariablePath.DriveName separately, which is why the issue does not affect environment variables despite VariableAnalysis.IsGlobalOrEnvironment erroneously returning false in some cases.

PR Checklist

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• [x] Summarized changes
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• [x] Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
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[liamjpeters]

Really clear explanation, thanks for getting to the root of the issue! 🙂 LGTM!