Feature : Change Newtonsoft.Json to System.Text.Json (#3562)

<!-- Please be sure to read the
[Contribute](https://github.com/reactiveui/reactiveui#contribute)
section of the README -->

**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

Feature #3190 

**What is the current behaviour?**
<!-- You can also link to an open issue here. -->

No support for System.Text.Json

**What is the new behaviour?**
<!-- If this is a feature change -->

System.Text.Json support for Serialization added

https://learn.microsoft.com/en-us/dotnet/standard/serialization/system-text-json/migrate-from-newtonsoft?pivots=dotnet-7-0

**What might this PR break?**

If other Serializers are used verification of correct serialisation
should be confirmed

**Please check if the PR fulfills these requirements**
- [x] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

Author: ChrisPulman

.editorconfig

@@ -296,7 +296,7 @@ dotnet_diagnostic.SA1006.severity = error
 dotnet_diagnostic.SA1007.severity = error
 dotnet_diagnostic.SA1008.severity = error
 dotnet_diagnostic.SA1009.severity = error
-dotnet_diagnostic.SA1010.severity = error
+dotnet_diagnostic.SA1010.severity = suggestion
 dotnet_diagnostic.SA1011.severity = error
 dotnet_diagnostic.SA1012.severity = error
 dotnet_diagnostic.SA1013.severity = error
@@ -525,3 +525,5 @@ indent_size = 2
 end_of_line = lf
 [*.{cmd, bat}]
 end_of_line = crlf
+
+vsspell_dictionary_languages = en-US

src/Directory.build.props

@@ -13,10 +13,11 @@
     <DefaultPackageDescription>A MVVM framework that integrates with the Reactive Extensions for .NET to create elegant, testable User Interfaces that run on any mobile or desktop platform. Supports Xamarin.iOS, Xamarin.Android, Xamarin.Mac, Xamarin Forms, Xamarin.TVOS, Tizen, WPF, Windows Forms, Universal Windows Platform (UWP) and the Uno Platform.</DefaultPackageDescription>
     <PackageDescription>$(DefaultPackageDescription)</PackageDescription>
     <Owners>xanaisbettsx;ghuntley</Owners>
-    <PackageTags>mvvm;reactiveui;rx;reactive extensions;observable;LINQ;events;frp;xamarin;android;ios;mac;forms;monodroid;monotouch;xamarin.android;xamarin.ios;xamarin.forms;xamarin.mac;xamarin.tvos;wpf;net;netstandard;net461;uwp;tizen;unoplatform</PackageTags>
+    <PackageTags>mvvm;reactiveui;rx;reactive extensions;observable;LINQ;events;frp;xamarin;android;ios;mac;forms;monodroid;monotouch;xamarin.android;xamarin.ios;xamarin.forms;xamarin.mac;xamarin.tvos;wpf;net;netstandard;net462;winui;maui;tizen;unoplatform</PackageTags>
     <PackageReleaseNotes>https://github.com/reactiveui/ReactiveUI/releases</PackageReleaseNotes>
     <RepositoryUrl>https://github.com/reactiveui/reactiveui</RepositoryUrl>
     <RepositoryType>git</RepositoryType>
+    <NoWarn>$(NoWarn);VSSpell001</NoWarn>
 
     <!-- Publish the repository URL in the built .nupkg (in the NuSpec <Repository> element) -->
     <PublishRepositoryUrl>true</PublishRepositoryUrl> 
@@ -33,6 +34,9 @@
     <IncludePackageReferencesDuringMarkupCompilation>true</IncludePackageReferencesDuringMarkupCompilation>
     <PublishRepositoryUrl>true</PublishRepositoryUrl> 
   </PropertyGroup>
+  <PropertyGroup Condition="$(IsTestProject) != 'true'">
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+  </PropertyGroup>
 
   <!-- MonoAndroid doesn't seem to want to allow debugging for maintainers -->
   <PropertyGroup Condition=" $(TargetFramework.StartsWith('MonoAndroid')) ">