Refactor SlnFileFactory + slnf

[edvilme]

Updates SlnFileFactory to provide a centralized way of finding, listing and parsing solution files using vs-solutionpersistence, including solution filter files (.slnf)

This includes tentative support for dotnet sln file.slnf list

[edvilme]

Some notes/comments

• Support for slnf is added in such a way that it is easy to replace in case it is added to vs-solutionpersistence (Add support for Solution Filters (.slnf) microsoft/vs-solutionpersistence#85)
• Support for finding slnf files is turned off by default and can be turned on for each command that wants to support it, as to avoid unexpected behaviors. Though I am happy to explore or discuss this further: long term plan I think is supporting all three (.sln, .slnx, and .slnf)
• Implementation here is mostly similar to MSBuild's (/~https://github.com/dotnet/msbuild/blob/8d395fd2729ad43428252f37656eb5c97a180987/src/Build/Construction/Solution/SolutionFile.cs#L610-L643) except that validation is done here to ensure the listed projects do exist in the original solution

[edvilme]

Rebased as this was targeting release/9.0.2xx instead of release/9.0.3xx