v4: Horizontal collapse

[mdo]

Backports #34253 to v4. Changes the collapse-horizontal to width for those who've secretly been using this feature for years in Bootstrap 4 but never saw official support.

[julien-deramond]

LGTM! 👌

IDK if anything must be done in the v5 migration guide. At the moment we have:

Collapse plugin now supports horizontal collapsing. — Add .collapse-horizontal to your .collapse to collapse the width instead of the height. Avoid browser repainting by setting a min-height or height.

Depending from which version you start and to which version you go you have several scenarios.

• v4 (<v4.7.0) → v5 (>=v5.1.0): Collapse plugin now supports horizontal collapsing
• v4.7.0 → v5.0.0: Collapse plugin doesn't exist anymore
• v4.7.0 → v5 (>=v5.1.0): .width must be changed to .collapse-horizontal