Escape terms stem and pass in xref

If the terms stem or pass are used in an xref, Antora will interpret these as a passthrough instead of an xref. These terms (appearing alone or at the end of the name) need to be escaped when rewriting xrefs.

Example:

xref:stem:index[text]

should be:

xref:\stem:index[text]

Alternately, the colon can be replaced with an attribute reference, which will disrupt the passthough match (which happens before attribute references are resolved).

The escape is only required if the xref has a non-empty attrlist.