Source generator samples

[am11]

In .NET 6 and upcoming .NET 7, new source generators were added in runtime libraries. Given the other useful runtime related samples in this repo (https://github.com/dotnet/samples/tree/main/core), it would be nice to have a sample per generator which demonstrates how to use it in standalone scenarios.

• JsonSerializableAttribute (net6.0)
• LoggerMessageAttribute (net6.0)
• LibraryImportAtrribute (net7.0)
• RegexGeneratorAttribute (net7.0)

cc @jkoritzinsky

[IEvangelist]

Thank you for this recommendation. I do plan on creating content for the Regex source generation, captured here: dotnet/docs#30289