Expose a type alias of the underlying JsonValue representation

[gusty]

Internally in Fleece this is JsonValue, but externally this is not always re-exported.

Only for Newtonsoft it is defined as JsonValue, other implementations needs to open their specific namespace (ie: open FSharp.Data) in order to have JsonValue available.

This is normally not a big issue, but it would be nice (and possible clearer) if Fleece expose a unique type alias that maps to the specific JsonValue (or JToken) of the underlying Json lib.

It could be called JsonValue or JValue.

[wallymathieu]

Sounds like a good thing!