WIP: Add custom event for path changes

[johnolek]

This PR adds a custom event that fires any time the path changes.

I'm not sure if this is the kind of thing you want to have in the codebase so I thought I would start this PR as a kind of discussion.

For the thing I'm working on I thought it would be interesting to be able to view a game from both players' perspectives at the same time. For this to be useful would require the two boards to be in sync. Having a simple custom event like this allows me to achieve that with a simple event listener:

    pgnViewer.div.addEventListener('pathChange', (event) => {
      reversedViewer.toPath(event.detail.path)
    })

synced.boards.mov

At first I tried to do this by adding click event listeners to the back/forward arrows, but that doesn't account for keyboard nav or clicking on moves in the list, and they also got cleared out after opening the menu since I think the buttons get re-rendered.

Anyway, if this seems like an ok idea to you, I was thinking it might be best to include some kind of abstraction around it, especially since there are one or two other things I might like to keep track of (like board size).

[ornicar]

The pgn-viewer is based on chessground, which uses callback functions for events.
I think we should stick to that for consistency. Adding a pathChange callback to the pgn-viewer would be useful.

[johnolek]

Good point! not sure why I didn’t think of that