Add support for Solution Filters

[dansiegel]

Is your feature request related to a problem? Please describe.

Solution Filters are highly useful for a variety of tasks particularly when working on a large codebase where you might need to just work with a small subset of the projects in the solution.

Describe the solution you'd like
When selecting a solution allow the selection of a .slnf file

Describe alternatives you've considered
I'm not re-creating and managing multiple solution files...

Additional context
A solution filter is an easy to work with json file that provides the file of the solution to use and the projects to include in the explorer.

{
  "solution": {
    "path": "PrismLibrary.sln",
    "projects": [
      "src\\Containers\\Prism.DryIoc.Shared\\Prism.DryIoc.Shared.shproj",
      "src\\Containers\\Prism.Unity.Shared\\Prism.Unity.Shared.shproj",
      "src\\Prism.Core\\Prism.Core.csproj",
      "src\\Prism.Events\\Prism.Events.csproj",
      "tests\\Containers\\Prism.Container.DryIoc.Tests\\Prism.Ioc.DryIoc.Tests.csproj",
      "tests\\Containers\\Prism.Container.Shared\\Prism.Ioc.Shared.shproj",
      "tests\\Containers\\Prism.Container.Unity.Tests\\Prism.Ioc.Unity.Tests.csproj",
      "tests\\Prism.Core.Tests\\Prism.Core.Tests.csproj"
    ]
  }
}