You'll need to update the documentation to reflect this new feature.
Would also be good to find a way to test when the feature is disabled to ensure it's behaving properly. Also it's running well on older version of .net/.netcore without the support for those APIs.
PR Type
What kind of change does this PR introduce?
What is the current behavior?
.NET Core 3.0 has a new API for json serialization (see https://devblogs.microsoft.com/dotnet/try-the-new-system-text-json-apis/) but it's not supported by Uno.Immutables...
What is the new behavior?
System.Text.Json
API is supported exactly like the well knownNewtonsoft.Json
PR Checklist
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