Use `PublishUrl` as default value for `PublishDir` in .NET SDK

[dsplaisted]

Currently, publishing with the Web SDK supports either PublishUrl or PublishDir, but the .NET SDK only supports PublishDir. This can be a problem as the VS tooling to create publish profiles uses PublishUrl.

So we should probably use PublishUrl as the default value for PublishDir.

See dotnet/aspnetcore#35134 (comment)

@dsplaisted I very much support this, and Thank You very much! :)