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/LibJS/Runtime/Intl/SegmentIterator.cpp

@@ -21,7 +21,7 @@ NonnullGCPtr<SegmentIterator> SegmentIterator::create(Realm& realm, Unicode::Seg
     // 4. Set iterator.[[IteratedString]] to string.
     // 5. Set iterator.[[IteratedStringNextSegmentCodeUnitIndex]] to 0.
     // 6. Return iterator.
-    return realm.heap().allocate<SegmentIterator>(realm, realm, segmenter, string, segments);
+    return realm.create<SegmentIterator>(realm, segmenter, string, segments);
 }
 
 // 18.6 Segment Iterator Objects, https://tc39.es/ecma402/#sec-segment-iterator-objects