Runescape Style Chat.

Orginal: https://github.com/tgstation/tgstation/pull/50608

code/modules/client/client_defines.dm

@@ -15,6 +15,8 @@
 	var/last_message	= "" //Contains the last message sent by this client - used to protect against copy-paste spamming.
 	var/last_message_count = 0 //contins a number of how many times a message identical to last_message was sent.
 	var/ircreplyamount = 0
+	/// Messages currently seen by this client
+	var/list/seen_messages
 
 		/////////
 		//OTHER//