Allow sequence constructor in extension instructions that are implemented with named templates

[kosek]

This is follow up of #168

I would like to extend https://qt4cg.org/specifications/xslt-40/Overview.html#invoking-templates-with-extension-instructions and allow sequence constructor inside extension instruction. I'm using such instructions in my code and it would be nice to be able to rewrite them to pure XSLT 4.0 code. Sequence constructor could be mapped to predefined parameter name. E.g.

<t:_>Hello world.</t:_>

Would be translated to

<xsl:call-template name="t:_">
  <xsl:with-param name="xsl:input">Hello world.</xsl:with-param>
</xsl:call-template>

[michaelhkay]

I'm inclined to follow the convention that the contained sequence constructor has the same effect as a select attribute. So if the template defines a select attribute, and its type is not boolean or string, then it also allows a contained sequence constructor as an alternative to the select attribute.