feat: page 'close' event

[JoelEinbinder]

Adds an event for when a page closes.

[aslushnikov]

What's the usecase for this?

[JoelEinbinder]

I run in headful mode, and want to respond to the user closing my app.

Right now I can do

browser.on('targetdestroyed', target => {
if (target === mypage.target()) doSomething();
});

But that felt pretty clunky.