Added NetCore3.1 target platform as long as NetStandard2.0

src/SharpGLTF.Core/SharpGLTF.Core.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>    
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netcoreapp3.1</TargetFrameworks>
     <AssemblyName>SharpGLTF.Core</AssemblyName>
     <RootNamespace>SharpGLTF</RootNamespace>
     <LangVersion>7.3</LangVersion>
@@ -26,7 +26,7 @@
     </AssemblyAttribute>
   </ItemGroup>  
   
-  <ItemGroup>
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netstandard2.0' ">
     <PackageReference Include="System.Text.Json" Version="4.7.2" />
   </ItemGroup>
 

src/SharpGLTF.Toolkit/SharpGLTF.Toolkit.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netcoreapp3.1</TargetFrameworks>
     <AssemblyName>SharpGLTF.Toolkit</AssemblyName>
     <RootNamespace>SharpGLTF</RootNamespace>    
     <LangVersion>7.3</LangVersion>