Extend marshaller support to generic types

[eiriktsarpalis]

FYI @AArnott

[codecov]

Codecov Report

Attention: Patch coverage is 90.74074% with 5 lines in your changes missing coverage. Please review.

Project coverage is 94.60%. Comparing base (dc4f608) to head (a309d9e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
.../PolyType.SourceGenerator/Helpers/RoslynHelpers.cs	86.36%	3 Missing ⚠️
src/PolyType.Roslyn/Helpers/RoslynHelpers.cs	50.00%	1 Missing ⚠️
...yType.SourceGenerator/Parser/Parser.ModelMapper.cs	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #105      +/-   ##
==========================================
- Coverage   94.62%   94.60%   -0.03%     
==========================================
  Files         189      189              
  Lines       11970    12006      +36     
  Branches     1562     1567       +5     
==========================================
+ Hits        11327    11358      +31     
- Misses        641      645       +4     
- Partials        2        3       +1     

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[AArnott]

Awesome. And it appears no change is required in Nerdbank.MessagePack to support this aside from updating the PolyType version.