Add /openurl command (#2926)

Usage: `/openurl <URL>`. Opens the provided URL in the browser.

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/util/IncognitoBrowser.hpp

@@ -5,6 +5,6 @@
 namespace chatterino {
 
 bool supportsIncognitoLinks();
-void openLinkIncognito(const QString &link);
+bool openLinkIncognito(const QString &link);
 
 }  // namespace chatterino