feat: Support msbuild Directory.build.props

[jeremylong]

• partially resolves MSBuild analyzer could (should?) check for props files as well #1948
• removes unnecessary interfaces

[aikebah]

Coding of interpolation is not in line with its documented behaviour (was previously already an error in interpolation method in the Model class).

[jeremylong]

@aikebah thank you for the review and comments. Everything should be good to go now - with the exception of the error handling. I will update that in a day or two before finalizing and merging this PR.

[aikebah]

@aikebah thank you for the review and comments. Everything should be good to go now - with the exception of the error handling. I will update that in a day or two before finalizing and merging this PR.

YW, always keen to try and find potential issues before features are introduced. Have made a habit of reading specs and checking whether code is matching specs (though reading the specs of the MSBuild documentation leaves and itchy feeling due to their free format open-endedness so that you can only 'try your best to conform to whatever Visual Studio happens to do').

I prefer the formal specs where you can really determine what is conformant and what not. I think your updated naive implementation is the best we can do (which covers the only usages that I would trust to reliably work now and in the future in my projects were I to be a dotnet developer with Visual Studio solutions projects given the documentation that Microsoft has put up for it)