WebContent+LibWeb+LibJS: Simplify injection of JS console globals

Instead of creating a new global object and proxying everything through
it, we now evaluate console inputs inside a `with` environment.

This seems to match the behavior of WebKit and Gecko in my basic
testing, and removes the ConsoleGlobalObject which has been a source of
confusion and invalid downcasts.

The globals now live in a class called ConsoleGlobalObjectExtensions
(renamed from ConsoleGlobalObject since it's no longer a global object).

To make this possible, I had to add a way to override the initial
lexical environment when calling JS::Interpreter::run(). This is plumbed
via Web::HTML::ClassicScript::run().

Userland/Services/WebContent/Forward.h

@@ -9,7 +9,7 @@
 namespace WebContent {
 
 class ConnectionFromClient;
-class ConsoleGlobalObject;
+class ConsoleGlobalEnvironmentExtensions;
 class PageHost;
 class WebContentConsoleClient;
 class WebDriverConnection;