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)

先閱讀 PowerShell/PSScriptAnalyzer#1277 中連結的討論，並找出內部的 VariableAnalysis 類別及其相依性。在考量 .NET Core 和 .NET Standard 編譯目標的同時，將 fork 的類別與 PowerShell 的版本進行比較。完成的標準是：在不破壞分析器的情況下，處理技術債務、已知的分析錯誤，以及相依性或框架限制。