feat(nuxt): Register nuxt route provider (#23794)

Registers a route provider for Nuxt in the client plugin, outside the
`__SENTRY_TRACING__` guard.

Nuxt installs the router before its plugins run, so the plugin can read
it straight off `nuxtApp` rather than waiting for a Vue app the way
`@sentry/vue` has to. Reuses `createVueRouteProvider` from #23553's
successor rather than reimplementing the vue-router resolve handling.

Because it sits outside the tracing guard, route parameterization
survives when tracing is tree-shaken away.

Part of #23556