Fully support additional projects

[sharwell]

Fixes #464

[sharwell]

I checked dotnet/roslyn-analyzers for compatibility and found the following:

• The (significant) API breaking changes in this pull request did not result in any compilation errors for the repository.
• Due to the limitations of the previous AdditionalProjects property, dotnet/roslyn-analyzers did not use it and instead added additional projects through the SolutionTransforms property. All current cases where SolutionTransforms is used to add additional projects worked through the new AdditionalProjects property without needing to continue using the SolutionTransforms property.

[sharwell]

I checked out DotNetAnalyzers/StyleCopAnalyzers and found similar results to above. The tests did not include any multi-assembly tests, but did set OutputKind and DocumentationMode through solution transforms. I converted the tests to the new properties of ProjectState and they passed as expected.