Add MuVis app by @Treata11

[Treata11]

A free & open-source music visualizer, written with modern Swift/SwiftUI APIs.

1. [x] Project URL: https://github.com/Keith-43/MuVis
2. [x] Update contents.json instead of README
3. [x] One project per pull request
4. [x] Screenshot included
5. [x] Avoid iOS or open-source in description as it is assumed
6. [x] Use this commit title format if applicable: Add app-name by @github-username
7. [x] Use approved format for your entry