Add support for deeply nested keys

[erikvullings]

Hi Stephan,

I'm using your great library in many of my projects, as it offers a very elegant and practical solution to adding translations to my project. However, sometimes it is practical to organise keys of a component in a nested structure, but this is only partially possible: one subkey is allowed, or alternatively, a property for pluralisation, but not both. Also many AI code editors (Claude, Gemini, CodeWiki) expect more deeply nested keys, using dot notation, e.g.

var messages = {
  EDITOR: {
    NODE: {
      NAME: 'Name',
      DESCRIPTION: 'Description',
      ICON: 'Icon',
    },
    EDGE: {
      DIRECTION: 'Direction',
      EFFECT: 'Effect',
    },
  }
};

var t = translate(messages);

// Accessing a deeply nested key
console.log(t('EDITOR.NODE.NAME')); // This is illustrative; the actual call might vary based on how you pass multiple subkeys.