Add tvix(/eval), a Nix language implementation

[tazjin]

Note that I pointed the link at the actual source repo, instead of our read-only Github mirror (tvlfyi/tvix).

[alilleybrinker]

Just so you know, the trade-off with non-GitHub repos is that the star count won't be auto-updated by the repo's python script, and instead needs to be manually updated periodically.