[Feature] Add missing properties in forum & thread channels (#2469)

* add `AppliedTags` property

* convert collections into immutable arrays

* remove "not supported" remark

* implement `ThreadChannelProperties`

* Add `DefaultSlowModeInterval` and `DefaultSlowModeInterval` properties to forum channels

* add `Moderated` property to `ForumTag``

* `ForumTag` inherits `ISnowflakeEntity`

* Fix `DiscordRestClient.GetChannelAsync` not getting forum channel

* a lot of changes

added:
- channel flags
- `ForumTagBuilder`
- imroved channel modification

* fixed a bug in forum tag emoji parsing

* inherit forum channel from `INesteeChannel`

* implement `INestedChannel` in forum channels

* Add `Flags` property to channels

* add iteraface for forum tags & add equality operators

* Add default reaction emoji property

* add support for modifing default reaction & some renaming

* add createForumChannelAsync to guild

* *fix resharper being a d... and moving code to next line*

* add a `ForumChannels` property

* Some fixes & add support for `default_sort_order`

* fix misleading comment

* fix #2502

* support creating post with applied tags

* fix xmldoc

* set category id on model update

* add limit checks for tag count

src/Discord.Net.Rest/Entities/Guilds/RestGuild.cs

@@ -710,6 +710,19 @@ namespace Discord.Rest
         public Task<RestCategoryChannel> CreateCategoryChannelAsync(string name, Action<GuildChannelProperties> func = null, RequestOptions options = null)
             => GuildHelper.CreateCategoryChannelAsync(this, Discord, name, options, func);
 
+        /// <summary>
+        ///     Creates a category channel with the provided name.
+        /// </summary>
+        /// <param name="name">The name of the new channel.</param>
+        /// <param name="func">The delegate containing the properties to be applied to the channel upon its creation.</param>
+        /// <param name="options">The options to be used when sending the request.</param>
+        /// <exception cref="ArgumentNullException"><paramref name="name" /> is <see langword="null"/>.</exception>
+        /// <returns>
+        ///     The created category channel.
+        /// </returns>
+        public Task<RestForumChannel> CreateForumChannelAsync(string name, Action<ForumChannelProperties> func = null, RequestOptions options = null)
+            => GuildHelper.CreateForumChannelAsync(this, Discord, name, options, func);
+
         /// <summary>
         ///     Gets a collection of all the voice regions this guild can access.
         /// </summary>
@@ -1370,6 +1383,9 @@ namespace Discord.Rest
         /// <inheritdoc />
         async Task<ICategoryChannel> IGuild.CreateCategoryAsync(string name, Action<GuildChannelProperties> func, RequestOptions options)
             => await CreateCategoryChannelAsync(name, func, options).ConfigureAwait(false);
+        /// <inheritdoc />
+        async Task<IForumChannel> IGuild.CreateForumChannelAsync(string name, Action<ForumChannelProperties> func, RequestOptions options)
+            => await CreateForumChannelAsync(name, func, options).ConfigureAwait(false);
 
         /// <inheritdoc />
         async Task<IReadOnlyCollection<IVoiceRegion>> IGuild.GetVoiceRegionsAsync(RequestOptions options)