PAModel - modernize to langVer 13

[joem-msft]

Now that we're expecting to address some Experimental layout bugs, we're going to improve the assembly to utilize modern coding practices.

• Updates the LangVersion to 13, matching Persistence lib
• Add reference to PolySharp to fill framework gaps
• Add reference to Persistence lib so we can start sharing code, e.g. TfmAdapters, TfmExtensions
• Addressed some low-hanging fruit language constructs available in latest C# language