devel/zls: add new port

Zig Language Server, or zls, is a language server for Zig. The Zig
wiki states that "The Zig community is decentralized" and "There is no
concept of 'official' or 'unofficial'", so instead of calling zls
unofficial, and I'm going to call it a cool option, one of many.

https://github.com/zigtools/zls