Switch to System.Text.Json to get rid of Newtonsoft.Json dependency

Author: exyi

src/CheckTestOutput.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFramework>netstandard2.0</TargetFramework>
+    <TargetFrameworks>netstandard2.0;netcoreapp3.0;net5.0;net6.0</TargetFrameworks>
 
     <Description>Simple helper which checks that output of a test matches a file. If not matching, just git staging the new file will accept the new version.</Description>
     <PackageId>CheckTestOutput</PackageId>
@@ -19,9 +19,12 @@
     <PackageReadmeFile>README.md</PackageReadmeFile>
   </PropertyGroup>
 
+  <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
+    <PackageReference Include="System.Text.Json" Version="5.0.2" />
+  </ItemGroup>
+
   <ItemGroup>
-    <PackageReference Include="MedallionShell" Version="1.5.1" />
-    <PackageReference Include="Newtonsoft.Json" Version="12.0.1" />
+    <PackageReference Include="MedallionShell" Version="1.6.2" />
 
     <None Include="../LICENSE" Pack="true" PackagePath=""/>
     <None Include="../README.md" Pack="true" PackagePath=""/>

src/JsonChecks.cs

@@ -2,7 +2,8 @@
 using System.Collections.Generic;
 using System.IO;
 using System.Linq;
-using Newtonsoft.Json;
+using System.Text.Json;
+using System.Text.RegularExpressions;
 
 namespace CheckTestOutput
 {
@@ -16,17 +17,14 @@ public static void CheckJsonObject(
             [System.Runtime.CompilerServices.CallerMemberName] string memberName = null,
             [System.Runtime.CompilerServices.CallerFilePath] string sourceFilePath = null)
         {
-            var serializer = new JsonSerializer();
-            var outputString = new System.Text.StringBuilder();
-            using (var w = new JsonTextWriter(new StringWriter(outputString)))
-            {
-                w.Indentation = 1;
-                w.IndentChar = '\t';
-                w.Formatting = Formatting.Indented;
-                serializer.Serialize(w, output);
-            }
+            var strOutput =
+                JsonSerializer.Serialize(output, new JsonSerializerOptions() { WriteIndented = true });
+
+
+            // indent using tabs for back compatibility
+            strOutput = Regex.Replace(strOutput, "^(  )+", m => new string('\t', m.Value.Length / 2), RegexOptions.Multiline);
             t.CheckOutputCore(
-                outputString.ToString(),
+                strOutput,
                 checkName,
                 $"{Path.GetFileNameWithoutExtension(sourceFilePath)}.{memberName}",
                 fileExtension