IRCClient: Add is_channel_prefix to check if string is a channel name

Author: https://github.com/bcoles
Commit: 6bf47252dc
Pull-request: https://github.com/SerenityOS/serenity/pull/1580
Reviewed-by: https://github.com/awesomekling

Applications/IRCClient/IRCClient.h

@@ -60,7 +60,8 @@ public:
     void part_channel(const String&);
     void change_nick(const String&);
 
-    bool is_nick_prefix(char) const;
+    static bool is_nick_prefix(char);
+    static bool is_channel_prefix(char);
 
     IRCWindow* current_window() { return aid_get_active_window(); }
     const IRCWindow* current_window() const { return aid_get_active_window(); }