[#70200] Introduce Fieldset component, input group

[myabc]

Ticket

https://community.openproject.org/work_packages/70200

What are you trying to accomplish?

Introduces both an unstyled, low-level FieldsetComponent and the fieldset_group input group for use with the Primer Forms DSL.

Screenshots

See #21365 for fieldset_group in action.

Unstyled component (see lookbook docs)

What approach did you choose and why?

As with other recent components I've worked on (e.g. DataTable), i've chosen to introduce a low-level unstyled / "headless" component that just renders the HTML and validates semantics.

Merge checklist

• [x] Added/updated tests
• [x] Added/updated documentation in Lookbook (patterns, previews, etc)
• [x] Tested major browsers (Chrome, Firefox, Edge, ...)

[Copilot]

@myabc I've opened a new pull request, #21603, to work on those changes. Once the pull request is ready, I'll request review from you.