Performance: Split large dependencies into their own individual chunks

Created by: umpox

Splits some of our large dependencies that are unlikely to change into their own bundle. This means users can cache them for longer and would not have to re-download if some unrelated code also changed.

React: This has previously been included in our main app.bundle.js, but the content hash for this file will change regularly. We don't often update React so users can cache it for longer by moving it into a different bundle.

Closes https://github.com/sourcegraph/sourcegraph/issues/24883