textproc/libxml2: Update to 2.9.13 and migrate to CMake

Depend on ICU and (lib)readline to follow other distros

Changelog: https://gitlab.gnome.org/GNOME/libxml2/-/tags/v2.9.13

I'd like to thank both tcberner and mandree for reviewing,
suggesting improvements and helping out in general. I'd also like
thank antoine for doing exp-runs.

PR: 262288
Reviewed by: tcberner, mandree
Approved by: desktop (tcberner)
Differential Revision: https://reviews.freebsd.org/D34338
Exp-run by: antoine