vendor_stream_wrapper is slow

Problem/Motivation

debugbar assets are loaded through vendor_stream_wrapper but it is quite slow. It can take several seconds for each asset to load.
These assets are loaded at the end of the HTML so it is not a huge problem but it can delay the load of other assets that further down the queue.

Steps to reproduce

1. Enable debugbar.
2. Open the network tab of your browser dev tools.
3. See that each debugbar asset takes several seconds to load.

Proposed resolution

A solution could be to inline these assets.
We could also copy them from vendor/ to the Drupal root but this would probably require a manual or Composer post-install step.