Add lastmod

[starskyreverie]

Google recently deprecated the sitemap ping (1-6-2023) in favor of the <lastmod> element. Any chance that could be integrated?

https://developers.google.com/search/blog/2023/06/sitemaps-lastmod-ping

[mstibbard]

This appears infeasible to implement properly because svelte-sitemap would need to analyse the prerendered pages for material changes to the content. The linked page says:

... when we say "last modification", we actually mean "last significant modification". If your CMS changed an insignificant piece of text in the sidebar or footer, you don't have to update the lastmod value for that page. However if you changed the primary text, added or changed structured data, or updated some links, do update the lastmod value.

Google states they will ignore your use of the field if you misuse lastmod with not-significant-enough changes:

.. it needs to consistently match reality: if your page changed 7 years ago, but you're telling us in the lastmod element that it changed yesterday, eventually we're not going to believe you anymore when it comes to the last modified date of your pages.