HTMLLinkElement is the final user of Resource/ResourceClient (used for
preloads and icons). This ports these link types to use fetch according
to the spec.
Preloads were particularly goofy because they would be stored in the
ResourceLoader's ad-hoc cache. But this cache was never consulted for
organic loads, thus were never used. There is more work to be done to
use these preloads within fetch, but for now they at least are stored
in fetch's HTTP cache for re-use.
This is required to store Content Security Policies, as their
Directives are implemented as subclasses with overridden virtual
functions. Thus, they cannot be stored as generic Directive classes, as
it'll lose the ability to call overridden functions when they are
copied.
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
Now that the heap has no knowledge about a JavaScript realm and is
purely for managing the memory of the heap, it does not make sense
to name this function to say that it is a non-realm variant.
