Implement break-on-event

[captainbrosset]

I couldn't find an issue about this already.

Here is what the feature looks like in the old Firefox debugger:

You get a sidebar panel that lists all current event listeners, and you can click checkboxes to pause in these listeners when events happen.

Do you intend to have something somewhat similar to this at some stage in the new debugger?
It is very useful to discover a code base you don't know about.

[wldcordeiro]

We have an MVP of this from what I can tell, but there are issues with it.

[jasonLaster]

We have a proof of concept, which you can turn on with the flag eventListeners.

moving this to the enhancements board as it is a post-launch feature
https://github.com/devtools-html/debugger.html/projects/5

[janodvarko]

Meta bug for this feature in Bugzilla
https://bugzilla.mozilla.org/show_bug.cgi?id=1526079

Honza