LibWeb+WebDriver: Support running headless WebDriver sessions

This adds an "extension capability" for clients to indicate that a
headless browser should be used for the session.

Userland/Libraries/LibWeb/WebDriver/Capabilities.h

@@ -54,6 +54,12 @@ constexpr UnhandledPromptBehavior unhandled_prompt_behavior_from_string(StringVi
     VERIFY_NOT_REACHED();
 }
 
+struct LadybirdOptions {
+    explicit LadybirdOptions(JsonObject const& capabilities);
+
+    bool headless { false };
+};
+
 Response process_capabilities(JsonValue const& parameters);
 
 }