Implement IProjectFileWriter in a class

[Reokodoku]

Is your feature request related to a problem? Please describe.
To decompile an assembly I am trying to use only the decompiler library of ILSpy. If necessary I override methods, I create classes, etc... but in the WholeProjectDecompiler I cannot create a custom .csproj file, because I cannot create a class that implements IProjectFileWriter (and pass it to the constructor) nor override the two existing classes that implements the interface (because both have the sealed modifier).

I tried to modify the decompiler’s source code myself but randomly the decompilation time increased a lot.

Describe the solution you'd like
It would be convenient to make the interface IProjectFileWriter public and add a parameter with the interface type to the WholeProjectDecompiler constructor to pass the desired class.