Add scrape and rewrite rules for webtoons

[DIzFer]

Although the only source I have for the rewrite rule is, in fact, #892, it does work when combined with add_dynamic_image and scraping the right element. I have not investigated further.

Works around #775 and #1871 (as in, gives us working webtoons feeds but referer spoofing would still be a nice tool to have).

Fixes #256.

Do you follow the guidelines?

• ~I have tested my changes - I've tested the rules by manually adding them to feeds. I've not rebuilt the whole program, but syntax is identical to similar entries in both lists.
• I read this document: https://miniflux.app/faq.html#pull-request