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/Fetch/BodyInit.cpp

@@ -55,7 +55,7 @@ WebIDL::ExceptionOr<Infrastructure::BodyWithType> extract_body(JS::Realm& realm,
     }
     // 4. Otherwise, set stream to a new ReadableStream object, and set up stream with byte reading support.
     else {
-        stream = realm.heap().allocate<Streams::ReadableStream>(realm, realm);
+        stream = realm.create<Streams::ReadableStream>(realm);
         Streams::set_up_readable_stream_controller_with_byte_reading_support(*stream);
     }