Add support for the <podcast:trailer> tag

Problem

There is a trailer tag in the podcast namespace that allows trailers to be promoted on a feed. Several Podcasting 2.0 apps already support this tag, including some Castopod competitors, and use it to help discovery in their apps.

Describe the solution you'd like

All episodes within a podcast that have a type of "trailer" will have a <podcast:trailer> tag created that sits at the channel level.

If a trailer is for a specific season, the trailer tag will reflect that.

Additional context

https://github.com/Podcastindex-org/podcast-namespace/blob/main/docs/1.0.md#trailer