What problem does this feature solve?

first of all, will notify graceful tips for user as if done. and can work better with unimport plugin like no-import~

What does the proposed API look like?

for example:

<template>
    <h1>{{ foo }}</h1>  <!-- expect get tips for user like: please export  foo in script-->
</template>
<script lang="ts" setup>
    // nothing
    // if use unimport plugin, will auto return foo in ctx~
</script>

May be can support api defineReturn like defineExpose?

defineReturn would completely defeat the purpose of script setup which allows you to use everything defined inside: you should explicitly import anything used import { foo } from './file'