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().
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