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/Segments.cpp

@@ -20,7 +20,7 @@ NonnullGCPtr<Segments> Segments::create(Realm& realm, Unicode::Segmenter const&
     // 3. Set segments.[[SegmentsSegmenter]] to segmenter.
     // 4. Set segments.[[SegmentsString]] to string.
     // 5. Return segments.
-    return realm.heap().allocate<Segments>(realm, realm, segmenter, move(string));
+    return realm.create<Segments>(realm, segmenter, move(string));
 }
 
 // 18.5 Segments Objects, https://tc39.es/ecma402/#sec-segments-objects