improve treatment of nested links

[pkra]

While AmerMathSoc/texml#198 could help a lot, we could still do a better job here -- either more like HTML parsing or by stripping inner links.

[pkra]

I guess this relates to #430

[pkra]

Following the F2F for AmerMathSoc/texml#198, it's more consistent to "flatten" nested links and emit a warning (since we can assume that texml will prevent them, too).

[pkra]

This will formally be a breaking change. (Thought it should not have any noticeable side effects.)

[pkra]

I'm leaning towards removing unnest.js entirely and taking the reverse approach, i.e., check the output for .closest('a') and respond.

That will simplify xref (which has a special case for toc-entry//xref from the earliest days because toc-entry generates a link).