LibWeb+WebContent+WebDriver: Implement Dismiss Alert

Author: https://github.com/trflynn89
Commit: 7cf2feb047
Pull-request: https://github.com/SerenityOS/serenity/pull/16091
Reviewed-by: https://github.com/linusg

Userland/Services/WebContent/WebDriverClient.ipc

@@ -43,6 +43,7 @@ endpoint WebDriverClient {
     add_cookie(JsonValue payload) => (Web::WebDriver::Response response)
     delete_cookie(String name) => (Web::WebDriver::Response response)
     delete_all_cookies() => (Web::WebDriver::Response response)
+    dismiss_alert() => (Web::WebDriver::Response response)
     take_screenshot() => (Web::WebDriver::Response response)
     take_element_screenshot(String element_id) => (Web::WebDriver::Response response)
 }