add multisn8 repository

[MultisampledNight]

The following points apply when adding a new repository to repos.json

• I ran ./bin/nur format-manifest after updating repos.json (We will use the same script in github actions to make sure we keep the format consistent)
• By including this repository in NUR, I confirm that any copyrightable content in the repository (other than built derivations or patches, if applicable) is licensed under the MIT license
• I confirm that meta.license and meta.sourceProvenance have been set correctly for any derivations for unfree or not built from source packages

Additionally, the following points are recommended:

• All applicable meta fields have been filled out. See https://nixos.org/manual/nixpkgs/stable/#sec-standard-meta-attributes for more information. The following fields are particularly helpful and can always be filled out:
  • meta.description, so consumers can confirm that that your package is what they're looking for
  • meta.license, even for free packages
  • meta.homepage, for tracking and deduplication
  • meta.mainProgram, so that nix run works correctly