fix: App Context missing in slots

[chocolateimage]

Describe your changes

When using slots like this example:

<script setup>
import { Select } from 'primevue'
import VueDatePicker from '@vuepic/vue-datepicker'
import '@vuepic/vue-datepicker/dist/main.css'

const date = ref()
</script>

<template>
    <VueDatePicker v-model="date">
        <template #left-sidebar>
            <Select />
        </template>
    </VueDatePicker>
</template>

The <Select> component would not get passed the appContext while shadow rendering which causes errors, especially on libraries like PrimeVue which require the appContext to use config properties.

Setting appContext after calling h() is what is described by Evan You: vuejs/core#2097 (comment)

Issue ticket number and link

#1092

Checklist before requesting a review

• I have performed a self-review of my code
• I have ensured that unit tests pass without errors
• If it is a new feature, I have added a new unit test