Thanks @vanillajonathan for the demo at #17496 (closed)!
This PR finally implements this longstanding request for a horizontal collapse. We already have the JS support, so we were missing the CSS for .width.collapsing and a child element with a fixed width.
Fixes #17496 (closed).
Preview: https://deploy-preview-34253--twbs-bootstrap.netlify.app/docs/5.0/components/collapse/#horizontal