Explore possibility of Source Generators for JSON-LD support

[warriordog]

This is to explore a possible implementation of JSON-LD based on interfaces and Roslyn Source Generators. If successful, this will resolve #2.

The first step is to model all AP / AS types as interfaces rather than classes. This accomplishes two key things:

• Resolves multiple inheritance (as with OrderedCollectionPage)
• Allows an application to include multiple extensions of the native types

And has two key flaws:

• You can't create an instance of an interface
• Library users would have to manually define an implementation for every type they use

Both flaws are dealbreakers, which leads to the second step - implementing a source generator. The source generator will automatically produce all necessary implementation classes at compile time. Since this runs at application compile time, rather than library compile time, the implementation class is guaranteed to have all fields supported by the application. Application users would just need to create an empty partial class for each type needed.

[warriordog]

Downside: can't model case where a type "may" include an extension

[warriordog]

Impossible to implement efficiently using the temporary rewrite described in #21. This item is therefore blocked indefinitely.