<Link>...</Link> vs. Link="..." for Common files

[jkotas]

Most libraries .csproj projects use <Link>...</Link> to link files from Common. There are a few that use Link="...". We should unify on one style.

[jkotas]

See #1247 (comment) for discussion of pros and cons of each option.

[danmoseley]

It would be nice if there was a heuristic (it has had this behavior since the C# project system was created in VS2002)
dotnet/project-system#5765

[ViktorHofer]

I prefer the attribute syntax over the element one here. We usually wrap additional attributes into multiple lines, which should help readability, e.g.: https://github.com/dotnet/runtime/blob/master/src/libraries/pretest.proj#L43-L48.