feature: add Nix Flake support

[mikeborodin]

This PR adds nix "flake" configuration so that Nix users can easily use fvm
We use devbox which is an abstraction on top of nix to declaratively manage project dependencies.
With this adding fvm to the project dev environment is as easy as running devbox add github:leoafarias/fvm
This will result in the "flake" added to your devbox.json (where all the dependencies are declared):

{
  "$schema": "https://raw.githubusercontent.com/jetify-com/devbox/0.12.0/.schema/devbox.schema.json",
  "packages": [
    "github:leoafarias/fvm",
  ],
  "shell": {
    "init_hook": [
       "fvm flutter doctor"
    ]
  }
}

The benefit is this file is checked into the source control and everyone on the team will have fvm installed once they are in this dev environment :)

Testing steps for this pr

# install devbox
devbox init
devbox add github:mikeborodin/fvm/1eccdd2b3235b5455c613241f9ae78c7f1de2ae6
devbox shell
fvm list

This is my first PR here so let me know if I missed something 😊

thanks

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