Update/standardize the internal custom `VariableAnalysis` classes

This addresses technical debt.

... the VariableAnalysis classes that were forked from PowerShell's own classes 4 years ago. Those classes were not updated since and have small bugs that leads to certain types of analysis not being possible. Unfortunately, the original maintainers have taken only a subset of it and added a lot of custom code to it. I once tried to just delete it and replace it with the code from PowerShell and some of the challenges were that SMA are very entangled with each other in terms of dependencies and it compiles against .Net Core, but PSSA compiles against .Net Standard.

(https://github.com/PowerShell/PSScriptAnalyzer/pull/1277#issuecomment-508929230)

Inizia leggendo la discussione collegata in PowerShell/PSScriptAnalyzer#1277 e identificando le classi interne di VariableAnalysis e le relative dipendenze. Confronta le classi forkate con le versioni di PowerShell, tenendo conto delle destinazioni di compilazione .NET Core e .NET Standard. Il lavoro è completo quando il debito tecnico, i bug di analisi noti e i vincoli relativi alle dipendenze o al framework sono stati risolti senza compromettere l'analizzatore.