Allow PublicApiAnalyzers to track internal APIs with `[Internal]` prefix

Tracking internal APIs can be useful in cases where IVT is used and compatibility between different versions of components is required.

For example:

• Project1 has a ProjectReference to Project2.
• Project2 exposes IVT to Project1
• Both projects produce NuGet packages
• If I make a change in internal API in Project2 that is called by Project1, that means a binary incompatibility is introduced.

Suggestion: all internal APIs, unless annotated with EmbeddedAttribute, can be tracked as an opt-in feature.

That way, Project2 is forced to add EmbeddedAttribute when not intended to be used by IVT, and otherwise breaking changes can be tracked more easily.