Remove remaining Newtonsoft referenes.

Lombiq.JsonEditor.Test.UI/Extensions/TestCaseUITestContextExtensions.cs

@@ -1,9 +1,9 @@
 using Atata;
 using Lombiq.Tests.UI.Extensions;
 using Lombiq.Tests.UI.Services;
-using Newtonsoft.Json.Linq;
 using OpenQA.Selenium;
 using Shouldly;
+using System.Text.Json.Nodes;
 using System.Threading.Tasks;
 
 namespace Lombiq.JsonEditor.Tests.UI.Extensions;
@@ -139,7 +139,7 @@ private static async Task TestTreeStyleModeAsync(this UITestContext context)
     private static void TestCodeStyleMode(this UITestContext context)
     {
         // This field is hidden, but its content reflects what's in the editor.
-        var editorContent = JObject
+        var editorContent = JsonNode
             .Parse(context.Get(By.XPath($"//input[@class='jsonEditor__input']").OfAnyVisibility())
             .GetValue());
 

Lombiq.JsonEditor/Views/JsonField.cshtml

@@ -3,13 +3,13 @@
 
 @{
     var name = (Model.PartFieldDefinition.PartDefinition.Name + Model.PartFieldDefinition.Name).ToCamelCase();
-    var jObject = JObject.Parse(Model.Value ?? "{}");
+    var safeJson = JsonNode.Parse(Model.Value ?? "{}")?.ToString() ?? "{}";
 }
 
 @* This exposes the field value to the client JavaScript. Re-parsing the JSON into JObject ensures that there is no
     dangerous content (such as JS code injection) in there, so it's safe to pass the value into Html.Raw(). *@
 <script>
     if (!('JsonField' in window) || !window.JsonField) window.JsonField = {};
     var fieldName = @name.JsonHtmlContent();
-    window.JsonField[fieldName] = @Html.Raw(jObject.ToString());
+    window.JsonField[fieldName] = @Html.Raw(safeJson);
 </script>