This patch adds a new kind of JS::Value, the empty value.
It's what you get when you do JSValue() (or most commonly, {} in C++.)
An empty Value signifies the absence of a value, and should never be
visible to JavaScript itself. As of right now, it's used for array
holes and as a return value when an exception has been thrown and we
just want to unwind.
This patch is a bit of a mess as I had to fix a whole bunch of code
that was relying on JSValue() being undefined, etc.
LibWeb now creates a WindowObject which inherits from GlobalObject.
Allocation of the global object is moved out of the Interpreter ctor
to allow for specialized construction.
The existing Window interfaces are moved to WindowObject with their
implementation code in the new Window class.
