JLink: never block the caller from adding an attribute

[romain-pm]

Part of #749. A developer must never be pushed off the component because it will not let them add an attribute.

Adopting the API on Jahia/jahia.com moved all 24 link sites, but only 5 could use <JLink>. The other 19 dropped to getLinkProps plus a hand-written <a {...anchor}> plus a state.navigable branch — three lines where the component would have been one. The site puts five analytics attributes on every link (data-element-url, -type, -text, -location, -name), and two of them are derived from the href and the label the component just computed and did not hand back.

• attributes: an open map spread onto the <a>, taking either a record or a function of the resolved link (({ href, label, isCurrent }) => Record<string, string>). Static values are already expressible; the function form is the whole point. React's typings do not model data-* on a component's props, so this is the only route.
• asChild, for a wrapper that is not a bare <a> — jahia.com's <CTA> renders a button-shaped thing. Next.js met the same need and called it passHref ("Forces Link to send the href property to its child"), still in the next/link 16.3.2 types.
• Derive the omit from the component's own props (Omit<AnchorHTMLAttributes<HTMLAnchorElement>, keyof JLinkProps>) rather than a hand-written list, so a prop added later cannot silently swallow an attribute. This is how Next composes both its components, and the image side got the same bug in a different place (JImage: never swallow an attribute the caller needs #762).

This is the single highest-value change on the link API: it is the difference between 5 and ~26 component call sites on one real site.