templater: add function that expands internal literal templates
This will be used when rendering nested formatter items with the default
template, e.g.
fm.nested('parents', tmpl='{rev}:{node|formatnode}', sep=' ')
^^^^^^^^^^^^^^^^^^^^^^^
the default item template
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}