Add XSLT instruction checking

[pgfearo]

• Check XSLT Namespace URI
• Check XSLT mode names
• Check XSLT key names
• Check attribute-set names
• Check XPath/XSLT function name references
• Resolve XSLT xsl:call-template names
• Resolve with-param names in xsl:call-template
• Check XSLT accumulator names
• Check for duplicate global variable declarations
• Check for duplicate function name/arity declarations
• Check exclude-result-prefixes attribute
• Check import/include hrefs resolve to files that exist
• Check for duplicate function parameter names
• Check for duplicate template parameter names
• Check for duplicate parameter names in xsl:iterate
• Check XSLT params in templates occur first
• Check XSLT params in functions occur first
• Check XSLT params in xsl:iterate occur first
• Check XSLT as attribute content - e.g. xs:string, map(*) etc.
• Check name attribute values (with no AVTs) are valid names
  • Allow xsl:attribute name="xml:space" etc
• Check escaped {{ and }} in AVT/TVT contexts are properly nested
• Check XSLT attribute values are non-empty (unless allowed - e.g. 'namespace')
• Schema-based checks
  • XSLT Instruction is allowed in context
  • Required attributes on XSLT instruction are present
  • Other attributes are not present
  • Values of XSLT attributes with enum types are checked