consider re-externalising Sitemap parsing library and look at dev's other tools

[leonstafford]

https://github.com/JanPetterMG

Forked own version, then brought functions into WP2Static due to how long it took to see PHP 8 support in the composer.json file for the library.

Need to reconsider pros/cons of using those as external libs, including the dev's Sitemap and Atom parsing libs....

pros keeping external

• not my responsibility to keep up to date with upstream

cons of external lib

• WordPress can poop the bed when version conflicts between my referenced version and another plugin's referenced version
• reliant on those devs to make changes, such as awaiting this PHP 8 semver support, else end up forking it and referencing my fork, then switching back later or keeping in sync with upstream