Add support for textDocument/documentHighlight for XML Schema types

[angelozerr]

Here a demo with this XML file:

<?xml version="1.1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema" targetNamespace="http://www.w3.org/2001/XMLSchema">
	<xs:element name="elt" />
	<xs:complexType name="aComplexType" >
		<xs:sequence>
			<xs:element ref="xs:elt" />
			<xs:group ref="xs:elt"></xs:group>
		</xs:sequence>
	</xs:complexType>
	<xs:element name="elt2" substitutionGroup="xs:elt" />

	<xs:group name="elt" >
		<xs:sequence></xs:sequence>
	</xs:group>
	
	
</xs:schema>