similar-origin window agents have the [[CanBlock]] flag set to false.
Achieve this by hooking up JS's concept with an agent to HTML::Agent.
For now, this is only hooked up to the similar-origin window agent
case but should be extended to the other agent types in the future.
Resulting in a massive rename across almost everywhere! Alongside the
namespace change, we now have the following names:
* JS::NonnullGCPtr -> GC::Ref
* JS::GCPtr -> GC::Ptr
* JS::HeapFunction -> GC::Function
* JS::CellImpl -> GC::Cell
* JS::Handle -> GC::Root
