[Feature] Add Message property and UpdateAsync() to IModalInteraction (#2645)

* initial commit

* better (?) comments

* whoops, accidentally commited this

src/Discord.Net.Core/Entities/Interactions/Modals/IModalInteraction.cs

@@ -1,3 +1,6 @@
+using System;
+using System.Threading.Tasks;
+
 namespace Discord
 {
     /// <summary>
@@ -9,5 +12,24 @@ namespace Discord
         ///     Gets the data received with this interaction; contains the clicked button.
         /// </summary>
         new IModalInteractionData Data { get; }
+
+        /// <summary>
+        ///     Gets the message the modal originates from.
+        /// </summary>
+        /// <remarks>
+        ///     This property is only populated if the modal was created from a message component.
+        /// </remarks>
+        IUserMessage Message { get; }
+
+        /// <summary>
+        ///     Updates the message which this modal originates from with the type <see cref="InteractionResponseType.UpdateMessage"/>
+        /// </summary>
+        /// <param name="func">A delegate containing the properties to modify the message with.</param>
+        /// <param name="options">The options to be used when sending the request.</param>
+        /// <returns>A task that represents the asynchronous operation of updating the message.</returns>
+        /// <remarks>
+        ///     This method can be used only if the modal was created from a message component.
+        /// </remarks>
+        Task UpdateAsync(Action<MessageProperties> func, RequestOptions options = null);
     }
 }