Browsers should also come packages with the most popular is libraries like jQuery, bootstrap, vue, d3, etc (not a real list, just off the top of my head)
That's an implementation detail. Several parts of Firefox and Chrome are implemented internally as extensions.
We already have CDNs so one site downloading a version of a library works for all. Maybe content-addressible dependencies could make that even better, for cross-CDN support?
No it doesn’t, CDNs now very specifically are not able to download it for all sites like they used to. Chrome saw to this.