[tabs] Add keepMounted prop to Tabs.Root for consistency with Accordion.Root

## Summary

Add a `keepMounted` prop to `Tabs.Root` for consistency with `Accordion.Root`.

`Tabs.Panel` already supports `keepMounted`, but there is no way to set it once for all panels from `Tabs.Root`.

## Motivation

This mainly improves API consistency between Accordion and Tabs.

Today, users can write ``, but need to repeat `keepMounted` on every `Tabs.Panel`. A root-level prop would make Tabs match Accordion and provide a convenient default for preserving panel state, avoiding remounts, and supporting measured or animated panel content.