Use System.Text.Json instead of Newtonsoft.Json

[YVbakker]

System.Text.Json has been steadily replacing Newtonsoft.Json. It's more performant, simpler to use and an official microsoft package: any reason not to switch?

[eNeRGy164]

AFAIK, all the (de)serialization of types is not pluggable in System.Text.Json.

[YVbakker]

what do you mean by pluggable serialization?

[eNeRGy164]

This can create concrete types instead of interfaces:
https://github.com/dendrodocs/dotnet-shared-lib/blob/main/src/DendroDocs.Shared/Json/ConcreteTypeConverter.cs

For example:
https://github.com/dendrodocs/dotnet-shared-lib/blob/8359117a84f9e0cb60a918bca3690da0adee90d6/src/DendroDocs.Shared/Descriptions/TypeDescription.cs#L41

[eNeRGy164]

Will have to fix some regressions (#22)