refactor: move JsonModelExt to PluralKit.Core (in individual model/patch files)

PluralKit.Core/Utils/JsonUtils.cs

@@ -0,0 +1,19 @@
+using System;
+
+namespace PluralKit.Core
+{
+    internal static class JsonUtils
+    {
+        public static string BoundsCheckField(this string input, int maxLength, string nameInError)
+        {
+            if (input != null && input.Length > maxLength)
+                throw new JsonModelParseError($"{nameInError} too long ({input.Length} > {maxLength}).");
+            return input;
+        }
+    }
+    
+    public class JsonModelParseError: Exception
+    {
+        public JsonModelParseError(string message): base(message) { }
+    }
+}