Add an analyzer for pubternal

[rynowak]

We want to create an analyzer that squiggles usage of pubternal code exposed by ASP.NET Core.

EF has an existing analyzer for this that we want to reship for Microsoft.AspNetCore.Analyzers.

[gfoidl]

dotnet/extensions#345 also introduced such an analyzer -- but I don't know if this one can be used here.

[rynowak]

Yeah, the logic for this will be similar, but it will ship in our main analyzers package. The goal is to warn everyone if they are using an unsupported API.

[Pilchie]

@rynowak do you think area-platform is appropriate here?

[rynowak]

That's probably the best area

[dougbu]

This was fixed when we migrated Internal.AspNetCore.Analyzers into this repo, wasn't it @pranavkm❔ That is, can this issue be closed❔