Allow xsl:analyze-string to use the results of fn:regex #2642

We have a new function fn:regex() to return a reusable compiled regular expression, but there is currently no way to use this in xsl:analyze-string.

I suggest a new attribute regex="expression" on xsl:analyze-string; the required type of the (xpath) expression is fn:regex-record, that is, the type of the output of fn:regex(). If the new attribute is present then @pattern and @flags must be absent.