Switch from {xml2} to a backend supporting XPath 2.0

[MichaelChirico]

          BTW, this at least partially seems related to our XPath1.0 restriction, it would be great to to have an XPath2.0+ backend but I don't see any R support for that. LLM turned up this C++ library [`XQilla`](https://sourceforge.net/projects/xqilla/) though, a long-term project could be to write an R package wrapping this with the same/similar API as {xml2}.

Originally posted by @MichaelChirico in #2365 (comment)

There are several places in our code base that could benefit from XPath 2.0 features.

There is no such backend yet available to R packages, so this is a long-term bookmark issue for the case that such a package should ever arise.

[MichaelChirico]

Also linking the upstream libxml2 issue about supporting recent XPath in our backing C library:

https://gitlab.gnome.org/GNOME/libxml2/-/issues/31

No movement really on that in >5 years so doesn't seem too hopeful, but worth checking in now and again.

[IndrajeetPatil]

Another much more labour-intensive approach would be to use a low-level library that does support XPath 2.0 or later, provide R bindings for it, and then use that in lintr:

https://www.saxonica.com/saxon-c/index.xml

I doubt we will do that anytime soon, but I wanted to note this option anyway.

[IndrajeetPatil]

In case anyone is interested :)

https://linuxiac.com/libxml2-maintainer-steps-down/