[Feature] Provide an utility to run NPM in a specific directory, with a specific node install

[Lucassifoni]

On systems that already have a Node install, leveraging an in-app npm install by Nodelix can raise issues as Npm will lookup PATH to find node. There was a npm flag allowing to provide a custom Node path but it has been removed.

One solution is to do runtime $PATH alteration like this :

def install_deps(project_path) do
    npm = Nodelix.VersionManager.bin_path(:npm, get_vsn())
    node_dir = Path.dirname(npm)

    System.cmd(npm, ["i"],
      cd: project_path,
      env: [{"PATH", "#{node_dir}:#{System.get_env("PATH")}"}]
    )
  end

But a more general utility would be nice to have.

[sheerlox]

I don't think this issue exists with the nodelix.npm Mix task because it calls the npm bin as a node bin argument. Providing a run_npm helper similar to this directly from the Nodelix module might be a good solution.

nodelix/lib/mix/tasks/nodelix.npm.ex

Lines 47 to 51 in 9574b5f

	version = opts[:version] || VersionManager.latest_lts_version()
	npm_path = VersionManager.bin_path(:npm, version)
	Mix.Tasks.Nodelix.run(["--version", version] ++ [npm_path | npm_args])

[Lucassifoni]

Nice, building on that to provide an npm command runner to the user would be great. I'll refactor in this direction