Add support for importScripts()

The implementation could be relatively simple:

```js
const vm = require('vm');
const fs = require('fs');
function importScripts() {
  for (var i=0; i<arguments.length; i++) {
    var url = arguments[i];
    vm.runInThisContext(fs.readFileSync(url,'utf-8')),{ filename: url });
  }
}
```