.NET 6: remove the concept of binding projects and add support for binding native libraries/frameworks to normal library/executable projects

There's nothing special about binding projects that couldn't also be made to work from other projects (library / executable) projects, so to simplify things, we should just make that happen for .NET 6.

Some care would have to be taken with regards to default compile includes, since there will be *.cs files in the project directory which should be pre-processed, and not passed to csc like other *.cs files (ApiDefinition.cs / StructsAndEnums.cs, etc).

Scenarios (to add tests for):

* [ ] Creating a NuGet for a library project with a binding.
* [ ] Referencing a library project with a binding from an executable project.
* [ ] Referencing a library project with a binding in another library project.
* [ ] Adding a binding directly to an executable project.