Move fieldgroup CSS to its own library

I'm not sure what testing instructions are here.

The field cardinality form adds the class without using the element type, but because the template and CSS are different in Claro it can't get broken by the change anyway.

The installer site configure form is the only code in core that actually uses the element, but this is also overridden by Claro so again I'm not sure installing with it actually validates the change either. Even the minimal profile uses Claro in the installer.

Because it's almost but not quite unused in core, and there appear to be only a couple of usages in contrib, I opened 📌 Deprecate #type fieldgroup Active .

OK I found a way to test.

Start a clean install of standard, on the site information form in the installer, inspect HTML, and check that fieldgroup.module.css is added.

Can also disable CSS aggregation after install and ensure the file doesn't get loaded on a regular page (any page really because the element is just not used in runtime operation).

Seems to look the same as before to me.

nod_ → closed merge request !11440

Committed fae1be9 and pushed to 11.x. Thanks!

Automatically closed - issue fixed for 2 weeks with no activity.