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/FileAPI/FileReader.cpp

@@ -55,7 +55,7 @@ void FileReader::visit_edges(JS::Cell::Visitor& visitor)
 
 JS::NonnullGCPtr<FileReader> FileReader::create(JS::Realm& realm)
 {
-    return realm.heap().allocate<FileReader>(realm, realm);
+    return realm.create<FileReader>(realm);
 }
 
 JS::NonnullGCPtr<FileReader> FileReader::construct_impl(JS::Realm& realm)