[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

test/Discord.Net.Tests.Unit/MockedEntities/MockedCategoryChannel.cs

@@ -20,6 +20,8 @@ namespace Discord
         public DateTimeOffset CreatedAt => throw new NotImplementedException();
 
         public ulong Id => throw new NotImplementedException();
+        
+        public ChannelFlags Flags => throw new NotImplementedException();
 
         public Task AddPermissionOverwriteAsync(IRole role, OverwritePermissions permissions, RequestOptions options = null)
         {

test/Discord.Net.Tests.Unit/MockedEntities/MockedTextChannel.cs

@@ -34,6 +34,8 @@ namespace Discord
 
         public ulong Id => throw new NotImplementedException();
 
+        public ChannelFlags Flags => throw new NotImplementedException();
+
         public Task AddPermissionOverwriteAsync(IRole role, OverwritePermissions permissions, RequestOptions options = null)
         {
             throw new NotImplementedException();

test/Discord.Net.Tests.Unit/MockedEntities/MockedVoiceChannel.cs

@@ -36,6 +36,8 @@ namespace Discord
 
         public string Mention => throw new NotImplementedException();
 
+        public ChannelFlags Flags => throw new NotImplementedException();
+
         public Task AddPermissionOverwriteAsync(IRole role, OverwritePermissions permissions, RequestOptions options = null) => throw new NotImplementedException();
         public Task AddPermissionOverwriteAsync(IUser user, OverwritePermissions permissions, RequestOptions options = null) => throw new NotImplementedException();
         public Task<IAudioClient> ConnectAsync(bool selfDeaf = false, bool selfMute = false, bool external = false) => throw new NotImplementedException();