Rename JsonSerializable as JacksonSerializable in 3.0

[cowtowncoder]

(note: part of https://github.com/FasterXML/jackson-future-ideas/wiki/JSTEP-6)

Interface JsonSerializable is something that value classes may implement to define custom serialization as part of the value class itself. Due to historical reasons there is the "Json" prefix; but since this type and usage is format-agnostic it really should not be used.

For Jackson 3.0 we have some option; 2 leading ones being

• ValueSerializable (or maybe SerializableValue or even ValueSerialization) -- would be consistent with renaming in Rename JsonDeserializer/JsonSerializer as ValueDeserializer/ValueSerializer in 3.0 #3044.
• JacksonSerializable (or perhaps JacksonSerialiation) -- clearly indicates this is a Jackson-specific thing

While initially former seemed preferable for consistency, my current thinking is that latter is better since this type would be mostly used for value types that otherwise do not refer to Jackson types (except for methods defined by the interface). For code readability it seems more likely that the reference to Jackson is good in indicating specific reason for the interface in that context, and consistency with types not of immediate interest there is off less importance.