Add a package for libpsl-0.20.2. PR pkg/53459.

A "public suffix" is a domain name under which Internet users can
directly register own names.

Browsers and other web clients can use it to

* avoid privacy-leaking "supercookies" avoid privacy-leaking
* "super domain" certificates (see post from Jeffry Walton) domain
* highlighting parts of the domain in a user interface sorting
* domain lists by site

Libpsl...

* has built-in PSL data for fast access
* allows to load PSL data from files
* checks if a given domain is a "public suffix"
* provides immediate cookie domain verification
* finds the longest public part of a given domain
* finds the shortest private part of a given domain works with
  international domains (UTF-8 and IDNA2008 Punycode)
* is thread-safe
* handles IDNA2008 UTS#46

(bsiegert)