Use AST to determine SupportsShouldProcess when 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.

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
• 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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[bergmeister]

Code change looks ok but the build says it has a problem when targeting psv3, looks like we need a pragma using the PSV3 variable

UseShouldProcessCorrectly.cs(382,99): error CS1061: 'ExpressionAst' does not contain a definition for 'SafeGetValue' and no accessible extension method 'SafeGetValue' accepting a first argument of type 'ExpressionAst' could be found (are you missing a using directive or an assembly reference?)

[rjmholt]

Ok I've repurposed the SafeGetValue substitute method I wrote to handle that. It's now available as a Helper method.

[bergmeister]

Looks good to me. I verified that the diff in Helper.cs/Settings.cs is mainly just the move to make the method publicly accessible.
Only an optional style suggestion.