Crossref support for _tags_

[cscheid]

My addition here to the wish list for quarto crossref (@cscheid :-)): a mechanism for \tagging. For equations, one could keep LaTeX syntax, that is what presumably most users of such a feature already are familiar with.

Originally posted by @ute in #4136 (comment)

[cscheid]

We need to support regular crossref syntax rather than latex, and if we're going to support tagging, then we should support tagging for every possible kind of crossref.

I'm thinking that for quarto, a crossref tag is "just" some additional text associated with a crossref that allows the crossref label to be customized on a per-crossref basis.

[cscheid]

See also #1095, #1771, #2356, #2439, #3440.