Add "close" event

[christopherjanzen]

Maybe I'm missing a technique that would easily allow me to watch for this, but it would be nice to have a "close" event like the "open", "next", and "previous" that are already available.

Use case: updating a slider slide index on close to match the lightbox's last open slide.

[midzer]

@christopherjanzen Thanks for filling.

I think this just involves putting

// Create and dispatch a new event
const closeEvent = new window.CustomEvent('close')

lightbox.dispatchEvent(closeEvent)

at the end of close().

Wanna contribute yourself, @christopherjanzen ?