PS: Add autogenerated summary models

[MathiasVP]

This PR does lots of model-related stuff:

1. The first commit changes all our current type models to be lower case, and adds a bunch of autogenerated models identified by running the C# model generator on https://github.com/PowerShell/PowerShell. I would not recommend looking at this commit unless you're feeling cold in your room and wants your computer to raise the temperature 😅
2. The next commit adds a few more models and extends the MaD language to allow for a pipeline parameter similar to a this parameter.
3. The third commit adds taint-flow through all calls to toString. We could do this by adding lots of summary models, but I figured we might as well just adds it manually
4. The fourth commit adds a missing flow step I noticed: Given a pipeline such as x_1 | x_2 | ... | x_n we have flow from x_i to x_(i+1), but we didn't have flow from x_n to the resulting expression.
5. The fifth commit adds a few more models after fixing some of the inputs to the MaD library that was necessary after cd8e5e6
6. The last commit adds a test that demonstrates flow through Join-String (which is a MaD-provided summary that specifies taint from the pipeline parameter to the return value). And yay it works!