move @nx/devkit to peer deps

[ruscon]

Since this is an nx plugin, @nx/devkit may already be installed.
Just need to specify which versions of @nx/devkit are supported as peer deps.

For example:

{
  "peerDependencies": {
    "@nx/devkit": "^18.0.0 || ^19.0.0"
  }
}

In most cases, peer dependencies are installed automatically.