feat: Add support for importing external ES Modules

[VLTHellolin]

Resolve #180.

Pass the modules we want to externalise and their links to externalModules:

monkey({
  // ...
  build: {
    externalModules: {
      react: 'https://esm.sh/react@19.1.0',
      'react-dom': 'https://esm.sh/react-dom@19.1.0',
    },
  },
}),

import React, { useState } from 'react';
import ReactDOM from 'react-dom';

// ...

And the generated code will be as follows:

// ==UserScript==
// @name       esm support test
// @namespace  esm-support-test
// @version    1.0.0
// @author     VLTHellolin
// @match      https://www.google.com/
// ==/UserScript==

(async function () {
  'use strict';

  const require$$0 = await (import('https://esm.sh/react@19.1.0').then(m => m.default));
  const { useState } = await import('https://esm.sh/react@19.1.0');
  const require$$0$1  = await (import('https://esm.sh/react-dom@19.1.0').then(m => m.default));

  // ...

})();