[RSS bot] Support <comments>

Nobody has claimed this yet.

While `` is the main news URL, `` is the optional URL of the comments associated with the news. https://www.rssboard.org/rss-specification#ltcommentsgtSubelementOfLtitemgt

What about a variable `$COMMENTS` to expose it?

**Additional context**

Lemmy is like Reddit, a forum that associates a discussion thread with an optional (external, non-Lemmy) news URL. I have not checked Reddit yet, but in Lemmy
1. the URL of a discussion thread *with* a news goes into `` and the news URL goes into ``;
2. the URL of a discussion thread *without* a news goes into `` and ``.

This is an [explicit decision](https://github.com/LemmyNet/lemmy/issues/1378) at Lemmy, in light of the RSS spec linked above.

For example, today [this](https://feddit.it/c/openstreetmap_it) community has `È online l'Archivio...` for case 1 and `Sono stati pubblicati gli Atti...` for case 2, and its RSS is [here](https://feddit.it/feeds/c/openstreetmap_it.xml?sort=New).

**Use case**

At our community (OpenStreetMap Italia), we want Matrix to always see the discussion thread. It provides context and clarification, and the news URL itself may already have been mentioned in the chat anyway. The Lemmy interface makes it easy to reach the news article from the discussion thread (just click the thread title) but clearly the inverse is not possible (the news artcle is not aware of Lemmy).

No file or test is named in the issue. Start by locating the RSS bot's item parsing and variable-exposure entry points, then compare how the existing link value is handled with feeds that include . Done means the comments URL is available as $COMMENTS without changing the existing link behavior, with coverage for feeds with and without comments URLs.

Written by the indexing model from the issue text.