enh(useElementSize): track the size of an element

[max-nextcloud]

☑️ Resolves

• Fix size computation broken in fix(NcReferenceWidget): use requestAnimationFrame in observers #5561.
• Add a composable for observing the size of an element.

#5561 did not just add the nextTick but also changed the resize handler to

				width.value = widgetRoot.value?.contentRect.width ?? 0

Where widgetRoot is the ref to the root node.
Turns out contentRect is only defined on the entries handed to the resizeObserver.
But widgetRoot is the ref of an HTML Element.

This PR fixes this,
adds tests,
and exposes the functionalilty of observing an elements size
which we are also using in text.

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 3️⃣ Backport to next requested with a Vue 3 upgrade

[max-nextcloud]

/backport to next

[max-nextcloud]

/backport to next

[susnux]

Why not simply using: https://vueuse.org/core/useElementSize/#useelementsize

[max-nextcloud]

Funny... I had looked for something like that but failed to see it. I guess ctrl-f would have helped.