Fix highlight mentions not updating when username changes (#6723)

Reported-by: James Upjohn <jupjohn@jammeh.co.nz>
Reviewed-by: pajlada <rasmus.karlsson@pajlada.com>
Reviewed-by: Nerixyz <nerixdev@outlook.de>

src/providers/twitch/TwitchAccount.hpp

@@ -65,6 +65,10 @@ public:
     // Returns true if the value has changed, otherwise false
     bool setOAuthToken(const QString &newOAuthToken);
 
+    // Attempts to update the users username
+    // Returns true if the value has changed, otherwise false
+    bool setUserName(const QString &newUserName);
+
     bool isAnon() const;
 
     void loadBlocks();