chore: adds constant for member_fields

lib/mv/constants.ex

@@ -0,0 +1,9 @@
+defmodule Mv.Constants do
+  @moduledoc """
+  Module for defining constants and atoms.
+  """
+
+  @member_fields [:first_name, :last_name, :email, :birth_date, :paid, :phone_number, :join_date, :exit_date, :notes, :city, :street, :house_number, :postal_code]
+
+  def member_fields, do: @member_fields
+end