Improving how the @context property is serialized

[Turnerj]

Describe the feature

Currently the @context property is always serialized. Using our SchemaSerializer class, we keep the outer @context property but we replace all inner @context properties by a StringBuilder.Replace call which means we allocate the whole JSON output as a string twice.

Ideally, we need a way to conditionally serialize the @context property only if it is the root object of serialization.

How to actually implement this (especially in a future System.Text.Json world) remains unknown however it should still be considered. Potentially if we can provide a custom converter for the root-level object that can output that property and for any nested properties, simply not pass that converter down - this may give us our desired effect.