Create serialization solution for RPGCore

[fydar]

RPGCore utilises abstract types and interfaces for Modular behaviours, many of which will need to be serialised.

Previously I have utilised Newtonsoft.Json, but since Microsoft has introduced System.Text.Json. I'm going to support both serializers and allow for user-defined serializers as well.

Serialisation libraries should support:

• Polymorphism
• Simple string serializers