Include PocketLogger custom formatting code as a content file as part of the Microsoft.DotNet.Interactive.VisualStudio NuGet package

[shyamnamboodiripad]

This makes it possible for consumers of this package to utilize the same custom formatting for commands and events as the dotnet-interactive tool when logging these objects via PocketLogger.

Also includes changes to:

1. Make the code in Format.CustomizeLogString.cs self-sufficient.
2. Make it an error to construct DiagnosticsProduced with 0 diagnostics. This a follow up to the change in Remove DiagnosticLogEntryProduced event. #3003 that allows us to clean up a TODO in Format.CustomizeLogString.cs.