Update RELAX NG grammar for XSLT

[ndw]

This PR updates the RELAX NG grammar to be (more) consistent with the XSD grammar (and consequently more correct for XSLT 4.0)

1. Relaxes the definition of QNames so that prefixes declared with fixed-namespaces can be supported
2. Adds the fixed-namespaces attribute to xsl:stylesheet, xsl:transform, and xsl:package
3. Adds the xsl:switch element
4. Adds the xsl:array and xsl:array-member elements
5. Adds shallow-copy-all to the possible values for on-no-match
6. Adds separator to xsl:apply-templates
7. Removes select attribute from xsl:copy
8. Adds as attribute to xsl:sequence
9. Updated the declarations for xsl:accumulator-rule, xsl:array, xsl:array-member, xsl:attribute, xsl:catch, xsl:comment, xsl:map-entry, xsl:matching-substring, xsl:message, xsl:namespace, xsl:non-matching-substring, xsl:on-empty, xsl:on-non-empty, xsl:otherwise, xsl:param, xsl:processing-instruction, xsl:sequence, xsl:sort, xsl:value-of, xsl:variable, xsl:when, and xsl:with-param so that they accept either a select attribute or a sequence constructor

More changes may also be required. A comprehensive comparision of the RNC and XSD schemas is needed, #1584

[ndw]

The CG agreed to merge this PR without discussion at meeting 099.