The base URI of the stylesheet, which forms part of the static context
of the stylesheet. This is used for resolving any relative URIs appearing
within the stylesheet, for example in xsl:include and xsl:import
declarations, in schema locations defined to xsl:import-schema,
or as an argument to the document() or doc() function.
This base URI is used only if the input supplied to the Compile method
does not provide its own base URI. It is therefore used on the version of the
method that supplies input from a Stream. On the version that supplies
input from an XmlReader, this base URI is used only if the XmlReader
does not have its own base URI.