Add Vivix - JavaScript execution visualizer

[HenryOnilude]

Checklist:

• I've read and understood Contributing Guidelines.
• I've added the new resource at the end of its section.
• This resource is out there for a while, and actively maintained.
• This resource is popular enough and has at least a few hundred stars on GitHub.

---

Vivix is a free, open-source JavaScript execution visualizer that steps
through code instruction by instruction, showing heap mutations, call stack
operations, and async/await execution simultaneously. No account required,
MIT licensed. Recently launched low star count but actively maintained
and hitting real developer audiences as it has 5,000 unique visitors, 58 countries
in 30 days.