feat: allowlisting which variables can be undefined

[preynal]

This PR enables defining allowUndefined as an array of variables that will not throw even if the var is undefined. It doesn't change the behaviour when allowUndefined is true or false

{
  "plugins": [
    ["dotenv-import", {
      "allowUndefined": ["ALLOWED_UNDEFINED_VAR"]
    }]
  ]
}