Access the collection of general Entities, both external and
internal, defined in the DTD. For example, in:
<!doctype example SYSTEM "ex.dtd" [
<!ENTITY foo "foo">
<!ENTITY bar "bar">
<!ENTITY % baz "baz">
]>
The Entities map includes foo and bar, but not baz. It is promised that
only Nodes which are Entities will exist in this NamedNodeMap.
For HTML, this will always be null.
Note that "built in" entities such as & and < should be
converted to their actual characters before being placed in the DOM's
contained text, and should be converted back when the DOM is rendered
as XML or HTML, and hence DO NOT appear here.