[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/Channels/RestGuildChannel.cs

@@ -26,6 +26,9 @@ namespace Discord.Rest
         /// <inheritdoc />
         public ulong GuildId => Guild.Id;
 
+        /// <inheritdoc />
+        public ChannelFlags Flags { get; private set; }
+
         internal RestGuildChannel(BaseDiscordClient discord, IGuild guild, ulong id)
             : base(discord, id)
         {
@@ -62,6 +65,8 @@ namespace Discord.Rest
                     newOverwrites.Add(overwrites[i].ToEntity());
                 _overwrites = newOverwrites.ToImmutable();
             }
+
+            Flags = model.Flags.GetValueOrDefault(ChannelFlags.None);
         }
 
         /// <inheritdoc />