JSON based encoding should completely ignore unexported fields

[nhooyr]

slog.JSON should behave exactly like json.Marshal and then we should have slog.Reflect for using slog's reflection with slog structure tags.

[nhooyr]

Don't think we need slog.Reflect since that is the default.

Lets just make slog.JSON a type that wraps around another type and declares that the value wrapped should be encoded as if exactly with encoding/json

The function is kinda weird since it's really a type.