Implement event modifiers

[Rich-Harris]

This finishes the work started in #1641.

Adding legacy support for once and passive would be possible, but realistically it would entail some larger changes, so I've decided to kick the can down the road for now.

[Rich-Harris]

This doesn't address custom events or component events; for now, modifiers will be silently ignored. That can be addressed in follow-up work. For components I'm not sure it makes sense; that should probably result in an error.

For custom events, modifiers could be passed to the event definition:

export default {
  events: {
    foo(node, callback, modifiers) {
      // ...
    }
  }
};

We'd just need to figure out whether modifiers should be an array, object or set. Not going to worry about it now though