[v2.0] Add a "group" option to the Deferred component.

[crotanite]

Love the new Deferred component, already made some use of it, but it would be nice if you could just pass a "group" parameter instead of individual keys.

I've created an example of the deferred.ts file in vue below to demonstrate what I mean.

import { defineComponent } from 'vue'

export default defineComponent({
  name: 'Deferred',
  props: {
    data: {
      type: [String, Array<String>],
      default: null,
    },
    group: {
      type: [String],
      default: 'default',
    },
  },
  render() {
    let keys: string[] = []
    if(this.$props.data) {
        keys = (Array.isArray(this.$props.data) ? this.$props.data : [this.$props.data]) as string[]
    } else {
        const deferredProps = this.$page.deferredProps
        keys = deferredProps ? (deferredProps[this.$props.group] as string[]) : []
    }

    if (!this.$slots.fallback) {
      throw new Error('`<Deferred>` requires a `<template #fallback>` slot')
    }
    return keys.every((key) => this.$page.props[key] !== undefined) ? this.$slots.default() : this.$slots.fallback()
  },
})