[Feature] Default thread ratelimit in guild text channels (#2622)

* initial commit

* mocked bruh

test/Discord.Net.Tests.Unit/MockedEntities/MockedTextChannel.cs

@@ -10,6 +10,8 @@ namespace Discord
     {
         public bool IsNsfw => throw new NotImplementedException();
 
+        public int DefaultSlowModeInterval => throw new NotImplementedException();
+
         public ThreadArchiveDuration DefaultArchiveDuration => throw new NotImplementedException();
 
         public string Topic => throw new NotImplementedException();

test/Discord.Net.Tests.Unit/MockedEntities/MockedVoiceChannel.cs

@@ -9,6 +9,8 @@ namespace Discord
 {
     internal sealed class MockedVoiceChannel : IVoiceChannel
     {
+        public int DefaultSlowModeInterval => throw new NotImplementedException();
+
         public int Bitrate => throw new NotImplementedException();
 
         public int? UserLimit => throw new NotImplementedException();