LibJS+LibWeb: Use realm.create<T> instead of heap.allocate<T>

The main motivation behind this is to remove JS specifics of the Realm
from the implementation of the Heap.

As a side effect of this change, this is a bit nicer to read than the
previous approach, and in my opinion, also makes it a little more clear
that this method is specific to a JavaScript Realm.

Libraries/LibWeb/DOMURL/URLSearchParams.cpp

@@ -127,7 +127,7 @@ Vector<QueryParam> url_decode(StringView input)
 
 JS::NonnullGCPtr<URLSearchParams> URLSearchParams::create(JS::Realm& realm, Vector<QueryParam> list)
 {
-    return realm.heap().allocate<URLSearchParams>(realm, realm, move(list));
+    return realm.create<URLSearchParams>(realm, move(list));
 }
 
 // https://url.spec.whatwg.org/#urlsearchparams-initialize