Implemented missing method and typing notifier

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -207,8 +207,6 @@ namespace Discord.Rest
 
         //Typing
         public static IDisposable EnterTypingState(IChannel channel, BaseDiscordClient client)
-        {
-            throw new NotImplementedException(); //TODO: Impl
-        }
+            => new TypingNotifier(client, channel);
     }
 }

src/Discord.Net.Rest/Utils/TypingNotifier.cs

@@ -0,0 +1,44 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Discord.Rest
+{
+    internal class TypingNotifier : IDisposable
+    {
+        private readonly BaseDiscordClient _client;
+        private readonly CancellationTokenSource _cancelToken;
+        private readonly ulong _channelId;
+
+        public TypingNotifier(BaseDiscordClient discord, IChannel channel)
+        {
+            _client = discord;
+            _cancelToken = new CancellationTokenSource();
+            _channelId = channel.Id;
+            var _ = Run();
+        }
+
+        private async Task Run()
+        {
+            try
+            {
+                var token = _cancelToken.Token;
+                while (!_cancelToken.IsCancellationRequested)
+                {
+                    try
+                    {
+                        await _client.ApiClient.TriggerTypingIndicatorAsync(_channelId);
+                    }
+                    catch { }
+                    await Task.Delay(4500, token);
+                }
+            }
+            catch (OperationCanceledException) { }
+        }
+
+        public void Dispose()
+        {
+            _cancelToken.Cancel();
+        }
+    }
+}