EventEmitter polyfill export disallows extension

When I run `snowpack dev --polyfill-node`, I get an error for a class extending the events polyfill:

`TypeError: Class extends value [object Object] is not a constructor or null`

Snowpack is outputting this:

```javascript
class Foo extends polyfillNode_events {
```

But `polyfillNode_events` is not a function and looks like this:
```javascript
{
EventEmitter: ƒ EventEmitter()
default: ƒ EventEmitter()
}
```

I'm not sure if Snowpack should be handling this by outputting `polyfillNode_events.default` or the exports in this package should be changed.