Extend sitemap.xml with lastmod property for Publish

Use case or problem

When publishing a vault to the web, it automatically creates a sitemap. But it currently lacks a lastmod property.

Proposed solution

Since the lastmod property is used by search engines to determine how frequently to crawl a site, this is important for SEO.

Current workaround (optional)

There is no current workaround.

Related feature requests (optional)