fix: make router useLink link reactive

[Antti-Palola]

Description

If for example VCard to prop is evaluated as undefined and then changed, the changes were not be propagated to the href.

fixes #19300

Card link after enabling link (from undefined to page2)

Before	After

Markup:

<template>
  <v-app>
    <v-container>
      Click button to toggle link state on list item
      <v-breadcrumbs active-color="primary">
        <v-breadcrumbs-item active>Hello</v-breadcrumbs-item>
        <v-breadcrumbs-divider>/</v-breadcrumbs-divider>
        <v-breadcrumbs-item to="/page2">Page 2</v-breadcrumbs-item>
      </v-breadcrumbs>
      <v-btn @click="setLink">Set link</v-btn>
      <v-list>
        <v-list-item :to="listLink" color="green">{{ `Link to ${listLink?.name}` }}</v-list-item>
        <!-- <v-list-item :to="{name: 'page1'}">Working link</v-list-item> -->
      </v-list>
      <v-card :to="listLink">
        <v-card-title>Linked card</v-card-title>
        <v-card-text>{{ `Link to ${listLink?.name}` }}</v-card-text>
      </v-card>
    </v-container>
  </v-app>
</template>

<script>
  import { ref } from 'vue'
  export default {
    name: 'Playground',

    setup () {
      const listLink = ref(undefined)
      function setLink () {
        if (listLink.value) {
          listLink.value = undefined
        } else {
          listLink.value = { name: 'page2' }
        }
      }
      return {
        listLink,
        setLink,
      }
    },
  }
</script>

[Antti-Palola]

The breadcrumbs cypress test is failing locally for me as well but I updated the Playground example to contain a breadcrumb too and it's working as it's supposed to.

This happened because of nonexistent route from previous test. Fixed now.