Add support for auto-completion of XML entities

In our documentation, we include XML entities via the following markup:

[
  <!ENTITY % entities SYSTEM "generic-entities.ent">
    %entities;
]>

The file generic-entities.ent contains entities, but also include other entity files as well via the following syntax:

<!ENTITY % product-entities     SYSTEM      "product-entities.ent">
%product-entities;

Would it be possible to parse these files for entities and offer their complete and sorted list as an intellisense feature after entering &?

Start by examining the XML entity examples in generic-entities.ent and the referenced product-entities.ent files to understand the inclusion syntax. Trace the extension's completion entry point and determine how nested entity files should be parsed and exposed after '&'; done means a complete, sorted entity list is offered through IntelliSense.