Add @书社:parsed-by to parse results

This switches the parser to use a two‐stage parse, in which each node
is by default first processed in the `书社:parse` mode, which then
applies templates to the node. This provides a hook for selecting
certain kinds of elements, for example `<html:script>` elements, and
doing something to the result.

The “something” in this case is “making note of the parser which is
registered to that type on each result element by setting the
`@书社:parsed-by` attribute to its `@id`.

This setup also allows a reparsing of the parse result (in case new
`<html:script>` elements were produced by it); it is worth noting that
this could result in an endless loop if the `<html:script>` element is
not actually transformed by any parser.