Fix usePage() reactivity in Vue 2 adapter

[reinink]

Right now the usePage() hook in the Vue 2 adapter isn't actually reactive. The following doesn't work:

<script setup>
import { usePage } from '@inertiajs/vue2'
import { computed } from 'vue'

const page = usePage()

const appName = computed(() => page.props.appName)
</script>

This PR updates the Vue 2 adapter to use the same technique used in the Vue 3 adapter to solve this (see #1469).

The only issue here is that this requires Vue 2.7 features to fix this — the computed() and reactive() methods. Meaning anyone who is still using Vue 2.6 will need to also upgrade to Vue 2.7 when updating to the next release of Inertia. Given that this is the only way to solve this problem, I think that's a reasonable tradeoff.