Add Weasyl as a predefined site

[huntertur]

Hello LinkStack folks,

LinkStack currently has built-in support for multiple art-focused websites: Picarto, Itaku, and Fur Affinity, among others. In this pull request, I propose adding another one (Weasyl).

I haven't contributed to LinkStack before, so I based this merge request on previous pull requests I see in the project history. Maintainers, if you are OK with my proposal to add this, please let me know if you have any concerns or other comments about my changes.