Add missing IThreadUser interface (#2055)

* Implement

* Add IMentionable to RestThreadUser

* Rather move mentionable to interface for consistency.

* Further consistency

src/Discord.Net.Core/Entities/Users/IThreadUser.cs

@@ -0,0 +1,25 @@
+using System;
+
+namespace Discord
+{
+    /// <summary>
+    ///     Represents a Discord thread user.
+    /// </summary>
+    public interface IThreadUser : IMentionable
+    {
+        /// <summary>
+        ///     Gets the <see cref="IThreadChannel"/> this user is in.
+        /// </summary>
+        IThreadChannel Thread { get; }
+
+        /// <summary>
+        ///     Gets the timestamp for when this user joined this thread.
+        /// </summary>
+        DateTimeOffset ThreadJoinedAt { get; }
+
+        /// <summary>
+        ///     Gets the guild this thread was created in.
+        /// </summary>
+        IGuild Guild { get; }
+    }
+}