LibWeb: Implement navigator.webdriver

This is defined via the NavigatorAutomationInformation interface mixin
from the WebDriver spec: https://w3c.github.io/webdriver/#interface

Userland/Libraries/LibWeb/HTML/Navigator.h

@@ -39,6 +39,8 @@ public:
     // https://html.spec.whatwg.org/multipage/system-state.html#dom-navigator-pdfviewerenabled
     bool pdf_viewer_enabled() const { return false; }
 
+    bool webdriver() const;
+
     virtual ~Navigator() override;
 
 private: