Plugin Reload: Migrate to System.Text.Json

[Jorteck]

With the addition of the NUI API, an extra dependency was added to Newtonsoft.Json due to its simplicity for serializing polymorphic objects.

Unfortunately it has a few issues from sometimes holding hard dependencies to plugins, preventing AssemblyLoadContexts from being unloading and causing hot reloads to fail.

[Jorteck]

Should not start working on this until we have appropriate test coverage: #463

[Samuel-IH]

Just saw this!
While this is a long ways away, I'd like to mention that System.Text.Json may not solve the issue. I've used it in one of my plugins to help with hot reloading with no success. I certainly could be missing something in my setup, but this issue was pretty convincing to me: dotnet/runtime#65323

I ended up having to use Jil instead: https://github.com/kevin-montrose/Jil