Don't fail on unknown embed kinds

Closes #936

DiscordChatExporter.Core/Utils/Extensions/StringExtensions.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.Text;
 using System.Text.RegularExpressions;
 
@@ -29,6 +30,11 @@ public static class StringExtensions
     public static string ToDashCase(this string str) =>
         Regex.Replace(str, @"(\p{Ll})(\p{Lu})", "$1-$2");
 
+    public static T? ParseEnumOrNull<T>(this string str, bool ignoreCase = true) where T : struct, Enum =>
+        Enum.TryParse<T>(str, ignoreCase, out var result)
+            ? result
+            : null;
+
     public static StringBuilder AppendIfNotEmpty(this StringBuilder builder, char value) =>
         builder.Length > 0
             ? builder.Append(value)