Allow flags into the root of the CLI

[zoedsoupe]

Currently is only possible to use the flag/2 macro inside a defcommand/2 or subcommand/2 macro.

This should be valid too:

defmodule MyCLI do
  use Nexus.CLI

  flag :version do
    short :v
    description "Shows the version of the program"
  end
  
  @impl Nexus.CLI
  def handle_input(:version, %{value: true}) do
    IO.puts(version())
  end
end