Fix incorrect standard emoji rendering in plaintext

DiscordChatExporter.Core.Markdown/EmojiNode.cs

@@ -1,4 +1,6 @@
-namespace DiscordChatExporter.Core.Markdown
+using Tyrrrz.Extensions;
+
+namespace DiscordChatExporter.Core.Markdown
 {
     public class EmojiNode : Node
     {
@@ -8,6 +10,8 @@
 
         public bool IsAnimated { get; }
 
+        public bool IsCustomEmoji => Id.IsNotBlank();
+
         public EmojiNode(string lexeme, string id, string name, bool isAnimated)
             : base(lexeme)
         {

DiscordChatExporter.Core/Services/ExportService.TemplateModel.cs

@@ -112,7 +112,7 @@ namespace DiscordChatExporter.Core.Services
 
                     else if (node is EmojiNode emojiNode)
                     {
-                        buffer.Append($":{emojiNode.Name}:");
+                        buffer.Append(emojiNode.IsCustomEmoji ? $":{emojiNode.Name}:" : node.Lexeme);
                     }
 
                     else