Update Zine to work with latest Zig

[klumme]

Hi @kristoff-it! Hope you don't mind the drive-by pull request, please let me know if there's anything I should do to improve it!

I was playing around with Zine and noticed it didn't work with the latest version of Zig. As a learning project, I tried digging into why, and ended up updating the dependencies, along with the code in one place (to accommodate ComptimeStringMap being renamed to StaticStringMap).

Note: getting Zine to work also depends on this pull request by desttinghim being merged in your fork of cmark-gfm!

I tried zig build and zig build serve with the zine-ssg.io repo, and everything seemed to work fine. I found that zig build docgen didn't work, but I don't think it's related to this pull request (I may be mistaken).