Add p:expand-templates

[ndw]

Back in the 1.0 days, I proposed p:template to expand attribute value and text value templates on an input document. In Prague in 2019, we decided that wasn't necessary because we had AVT/TVT in p:inline.

I don't know what we were thinking, but I can't think of it now. Given an external document that contains value templates, I can't think of any (reasonable!) way to expand them. (The unreasonable way: construct a p:run step where the content is in a p:inline is, I assert, unreasonable)

I think we should put back the p:template step, perhaps better named p:expand-templates:

<p:declare-step type="p:expand-templates">
  <p:input port="source" content-types="xml html"/>
  <p:output port="result" content-types="xml html"/>
  <p:option name="variables" as="map(xs:QName,item()*)?"/>
</p:declare-step>