Make singleton mode actually a singleton!

[developit]

This should fix #1, since TypeScript can now be used like so:

// webpack.config.js
module.exports = {
  module: {
    rules: [
      {
        test: /\.worker\.(js|ts)x?$/i,
        use: [{
          loader: 'comlink-loader',
          options: {
            singleton: true
          }
        }]
      }
    ]
  }
}

// index.ts
import { greet } from './hello.worker.ts';

(async () => {
  console.log( await greet('dog') );
})();

// hello.worker.ts

export function greet(subject: string): string {
  return `Hello, ${subject}!`;
}