LibGC+Everywhere: Factor out a LibGC from LibJS

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

Libraries/LibWeb/HTML/PotentialCORSRequest.cpp

@@ -10,7 +10,7 @@
 namespace Web::HTML {
 
 // https://html.spec.whatwg.org/multipage/urls-and-fetching.html#create-a-potential-cors-request
-JS::NonnullGCPtr<Fetch::Infrastructure::Request>
+GC::Ref<Fetch::Infrastructure::Request>
 create_potential_CORS_request(JS::VM& vm, URL::URL const& url, Optional<Fetch::Infrastructure::Request::Destination> destination, CORSSettingAttribute cors_attribute_state, SameOriginFallbackFlag same_origin_fallback_flag)
 {
     // 1. Let mode be "no-cors" if corsAttributeState is No CORS, and "cors" otherwise.