Move Nix-related files to nushell/integrations

Is your feature request related to a problem? Please describe.

We recently set up a community maintained https://github.com/nushell/integrations to keep package manager and other integration files for Nushell projects there. The intent is to avoid cluttering our repositories with 3rd party tool files that the core team has to maintain.

Describe the solution you'd like

Move Nix-related files to https://github.com/nushell/integrations

Describe alternatives you've considered

Since the core team doesn't actively develop nufmt (yet?), it depends on the active nufmt developers if they want these in and maintain them. But once we scale up the development, we might move them, since that's the direction we've followed in Nushell itself and recently nupm.

Begin by inventorying the repository's Nix-related files and reviewing the target nushell/integrations repository to understand where they belong. No specific files or tests are named; done means the relevant files are moved there without leaving the nufmt repository cluttered with those integrations.