Add rule for unused namespaces

[MartinGC94]

Summary of the new feature
It would be nice with a rule to find and remove unused using namespace statements similar to what you can do in C# in editors like VS.
Cleaning up unused namespaces can slightly improve type resolution performance and of course it makes scripts more neat and clean.

Proposed technical implementation details (optional)
Use FindAll on the root AST to find all Asts of the following types: TypeExpressionAst, TypeConstraintAst, AttributeAst and CommandAst where the command name is New-Object and try to resolve the specified TypeNames with GetReflectionType. Compare the written typename with the resolved typename and see if part of the namespace is missing, and if so if that part is listed in the list of namespaces. If it is then that namespace is in use and should stay.

For New-Object we can't use GetReflectionType so I'm not sure how to handle that. Type accelerators and in the future type/namespace aliases should probably also be taken into consideration.

What is the latest version of PSScriptAnalyzer at the point of writing

1.22.0

[SydneyhSmith]

Thanks @MartinGC94 this could be a tricky one since its a dynamic thing which needs the environment/session state

[bergmeister]

I'd support this but limit to just namespaces built into .NET/PowerShell because otherwise PSSA would need to know about namespaces defined in your module or others and load it. One can technically do that by doing this before running PSSA cmdlet but that's not the case for the editor.