Add option to set a custom `lastmod` date

[chraebsli]

This PR adds an option to all pages to manually set a lastmod date.

If sitemap.lastmod is not set in a pages header, it takes the legacy $page->modified(). With this, the lastmod option can be overridden, for example if:

• a page file is edited, but only something minor like frontmatter or formatting
• a site is migrated from another CMS, the file timestamps are recent but the content has not changed
• data displayed by the page is changed but the page .md file has not been touched. For example, the page contains content sourced from a datasource, a modular child, or displays a gallery which changed.

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This is an enhanced version of @hughbris #98

closes #98, #99

[chraebsli]

@Karmalakas I made a new commit b67de61 with all changes from the code review

Sorry, it took some time until I did this 5 min fix. I had my final exam from school and was not coding anything in the last days.

[rhukster]

thanks