Previously, all `GC::Cell` derived classes were Weakable. Marking only
those classes that require this functionality as Weakable allows us to
reduce the memory footprint of some frequently used classes.
In #1537, determine_the_origin() changed to take
`Optional<URL::URL> const&` as first parameter, but it's passed
`Web::Fetch::Infrastructure::Response::url()`, which returns
`Optional<URL::URL const&>`. Ladybird does not have
SerenityOS/serenity#22870 (yet?), so this mismatch silently creates
a copy.
Change determine_the_origin() to take `Optional<URL::URL const&>`
instead. No behavior change, saves a copy, and is probably what
was originally intended.
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
