types(withDefaults): enhance read-only type

[zqran]

Props should also be read-only in the following two cases:

const props = withDefaults(defineProps<{ name?: string }>(), {})
props.name // readonly

const props = withDefaults(defineProps<{ name?: string }>(), {
  name: ''
})
props.name // readonly

[sxzz]

LGTM