Allow intercepting events from within shadow DOM.

[aearly]

Fixes #14.

• Adds an example of a link within a shadow DOM that is not intercepted in browsers without the native Navigation API.
• The polyfill now looks up ev.composedPath()[0] rather than ev.target to find links. ev.target will only be the containing web component -- it doesn't cross shadow DOM boundaries.

Checklist

• run yarn build
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[fabiancook]

Thanks! Makes sense and looks good.

Ran into this in tests... it might be though because of events being created outside of the DOM... so they don't have the full DOM API available

process unhandled rejection TypeError: ev.composedPath is not a function

I'll do a commit for this right now, would be good to change too:

export interface EventPrototype {
    target: ElementPrototype;
    composedPath?(): ElementPrototype[];
    defaultPrevented: unknown;
    submitter: Record<string, unknown>;
}

And use, assuming too that it could be possible to get no values returned as well (maybe if its a CustomEvent, has composedPath but isn't populated somehow?):

function getComposedPathTarget(event: EventPrototype) {
  if (!event.composedPath) {
    return event.target;
  }
  const targets = event.composedPath();
  return targets[0] ?? event.target; 
}