WebDriver: Implement GET /session/{id}/url endpoint

Author: https://github.com/AtkinsSJ
Commit: 096fe865c6
Pull-request: https://github.com/SerenityOS/serenity/pull/15504
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/linusg

Userland/Services/WebDriver/Session.h

@@ -35,6 +35,7 @@ public:
     ErrorOr<void> stop();
     ErrorOr<void, Variant<HttpError, Error>> delete_window();
     ErrorOr<JsonValue, HttpError> post_url(JsonValue const& url);
+    ErrorOr<JsonValue, HttpError> get_url();
     ErrorOr<JsonValue, HttpError> get_title();
 
 private: