Add a function or property to Scribunto/Lua to get the default category sort key of a Title object

Related is T131911, which is about exposing a different set of page_props information via Scribunto/parser functions

I dislike the idea of exposing through the parser any properties that can be set via the parser.

In T154346#2909203, @Jackmcbarn wrote:

I dislike the idea of exposing through the parser any properties that can be set via the parser.

Okay. Do you dislike the {{#ifexist:Page name}} ParserFunction parser function?

I want to be able to get and use a page's stored metadata value on a separate page, in this case using the subject-space page's default category sort key on its talk page. How would you recommend this be implemented? Would using Scribunto/Lua for this be acceptable to you? Are there other implementation options to discuss and consider?

In T154346#2909529, @MZMcBride wrote:

Do you dislike the {{#ifexist:Page name}} ParserFunction parser function?

No, since the output of the parser can't change whether or not a page exists. Basically, I don't like things that could cause paradoxes if called on themselves, or on other pages in a loop. (See also my response to T71441).

Cross-reference: https://www.mediawiki.org/wiki/Requests_for_comment/Accessing_page_properties_from_wiki_pages.

One thought brought up during discussion of T154738 was changing the default behavior of sorting talk pages so that talk pages sort by the subject-space's default category sort key. For example, in the case of the subject-space page "Barack_Obama", if this subject-space page had a default category sort key of "Obama, Barack", then the page "Talk:Barack_Obama" would automatically default to using this sort key, unless otherwise specifically overridden on the talk page via {{DEFAULTSORT:}}.