Summary:
Patches vega's event handling for Canvas' in order to support firing events on multiple marks simultaneously.
This fixes a bug where the hover time wouldn't update if you stayed hovered over a line, since before only the hover line hit box would receive the mouseover event.

Note, I used patch-package to do the vega patching. I'm not familiar enough with how the UI gets deployed in prod/staging to know if this will work, maybe @zasgar knows?

Test Plan: Tested in Dev

Reviewers: zasgar, philkuz, nserrino, michelle, #engineering

Reviewed By: philkuz, #engineering

Subscribers: zasgar

Differential Revision: https://phab.corp.pixielabs.ai/D5499

GitOrigin-RevId: d64d828faae511fac7d65aa40d6f8dfa0085fcbe