Mark Interceptors Generator with `[Experimental]` Attribute

[Flash0ver]

ExperimentalAttribute Class was added in .NET 8.0.
Provide a conditional compatibility type for the netstandard2.0 TFM, if the latest version of PolySharp does not emit that Attribute (yet).

Questions:

• Which DiagnosticId to use?
  • e.g. F0Gen0001
• Should a UrlFormat be set as well?
  • e.g. link to Roslyn C# Language Feature Status
  • e.g. link to Interceptors proposal