Use AST to determine if SupportsShouldProcess has been set if an error is thrown

[rjmholt]

PR Summary

Fixes #1217.

When PowerShell throws an error due to not being able to resolve an attribute type, we sift through the AST to determine whether SupportsShouldProcess has been set on a function.

Also cleans up some of the logic so it's not all nested ass.

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