Add support for textDocument/references for XML Schema types

[angelozerr]

Add support for textDocument/references https://microsoft.github.io/language-server-protocol/specification#textDocument_references for extension

One extension for XSD could find the references of xs:complexType (references by xs:element, etc) :

<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" elementFormDefault="qualified" xmlns="http://maven.apache.org/POM/4.0.0" targetNamespace="http://maven.apache.org/POM/4.0.0">
  <xs:element name="project" type="Model">
...
  </xs:element>
  <xs:complexType name="Model">

A Find References did in the "Model" of txs:complexType shoudl return xs:element

Here a references demo with XMLSchema.xsd

The references should work with other element like:

• xs:complexType
• xs:simpleType
• xs:element
• xs:group