[accordion][tabs] Consider making `value` a required prop

[mj12albert]

Currently it's possible to omit the value prop in accordion and tabs entirely, using the index (by position) implicitly as the value instead.

This causes a few issues:

• Unable to "truly" unmount a TabPanel when keepMounted={false} [tabs] keepMounted={false} doesn't mount children but still mounts the root element #1872
• Mis-timed accordion animations [Accordion] Opening animation isn't cancelled when a panel is initially open #1588
• Unable to SSR initially open accordion or initially active tab

(In Radix, value is required for both accordion and tabs)

[mj12albert]

Ariakit has unmountOnHide which has the same limitation with Tabs, it only unmounts the children: https://codesandbox.io/p/sandbox/cool-nobel-9fgp9q

[ImSingee]

Support this.

Omitting the value offers minimal advantages but can result in multiple issues. On one hand, it may cause the technical problems highlighted in this issue. On the other hand, it likely increases the risk of errors during code maintenance for both users and base-ui maintainers.

[benface]

Omitting the value offers minimal advantages

Personally, I think it's really nice for DX (I hate naming things, and explicitly setting the value to an index feels silly and error-prone). But, if there's no good way around the issues it causes, I agree that it's not worth it.