feat(finder): enhance youtube channel parsing with default playlists

[mtsz-pl]

This PR enhances the existing custom YouTube parsing. It consists of two parts:

1. using canonical URL

https://www.youtube.com/@rossmanngroup and https://www.youtube.com/c/Jan%C5%9Apiewak1 are both proper YouTube channel URLs, and Miniflux tries to parse them with findSubscriptionsFromYouTube, but fails - because it expects the URL's path to contain either /channel, /watch or /playlist.

Conveniently, YouTube uses <link rel="canonical" href="...">, and this canonical URL is exactly what the parser wants. findCanonicalURL extracts this exact URL. That way we prevent YouTube links to fall through to generic feed parser in findSubscriptionsFromWebPage.

2. displaying default channel playlists

YouTube has this undocumented feature, where it generates multiple convenience playlist for each channel, using some clever prefixes (details in this SO post).

Now that we catch all YouTube channels correctly, we can automatically display default playlists, making it easier to subscribe only to shorts, live streams, or popular videos.

I'm using this myself a lot, it would be very convenient not to have to remember all these prefixes, and enter them manually. And I assume it's not even obvious for most people such possibility exists.

Screenshot:

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