Defines an extension function, possibly inline
Syntax:
<xsl:script
  implements-prefix = ncname
  language = "ecmascript" | "javascript" | "java" | qname-but-not-ncname
  src = uri-reference
  archive = uri-references>
  <!-- Content: #PCDATA -->
</xsl:script>
    
    Partially supported by Saxon 6.2 for Java only