LibWeb+WebDriver: Add an IPC-transferable Web::WebDriver::Response class

This is essentially an ErrorOr<JsonValue, Web::WebDriver::Error> class.
Unfortunately, that ErrorOr would not be default-constructible, which is
required for the generated IPC classes. So this is a thin wrapper around
a Variant<JsonValue, Web::WebDriver::Error> to emulate ErrorOr.

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -440,6 +440,7 @@ set(SOURCES
     WebAssembly/WebAssemblyTablePrototype.cpp
     WebDriver/Error.cpp
     WebDriver/ExecuteScript.cpp
+    WebDriver/Response.cpp
     WebGL/WebGLContextAttributes.cpp
     WebGL/WebGLContextEvent.cpp
     WebGL/WebGLRenderingContext.cpp