[Feature]: RSS - sorting feed items when outputing feed

[xc2]

What problem does this feature solve?

Also I think the feed should rather be sorted in the opposite order (newest first)
Originally posted by @slorber in #828 (comment)

thought both inoreader and thunderbird sort articles based on the pubDate by default when i'm adding a subscription, we should make feed items ordered in newest first when outputing feed as i'm not sure if most of the rss readers will sort items well.

What does the proposed API look like?

1. sort feed items in descending order by pubDate
2. add sorting option for customization sorting