Set VisualFSharpFull as default startup project

[cartermp]

Today, the legacy FSharp.LanguageService project is the default project. This is clearly wrong.

I tried setting this as the default, inspired by dotnet/project-system#3589

However, I don't think it's going to be as easy for us. Things I tried:

1. Setting VisualFSharpFull as the first project defined in the solution file
2. Doing (1) and making its solution folder the first one defined in the folder section of the solution file
3. Doing (1) and (2) and removing what looks like the unnecessary goop the FSI projects define

No matter what I do, FSharp.VS.FSI ends up being the default (including after wiping the .vs folder on my machine).

Halp @davkean and @Pilchie

[Pilchie]

Could be an issue in the legacy F# project system?