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)

首先阅读 PowerShell/PSScriptAnalyzer#1277 中链接的讨论，并确定内部的 VariableAnalysis 类及其依赖项。在考虑 .NET Core 和 .NET Standard 编译目标的同时，将 fork 的类与 PowerShell 的版本进行比较。完成的标准是：在不破坏分析器的情况下，解决技术债务、已知的分析错误以及依赖项或框架限制。