transformer: `export {}` with `declare` declaration

[Boshen]

rolldown/rolldown#3048

[Dunqing]

Depends on #7278

[sapphi-red]

It seems this is used by vitepress's build-time data loading feature. Maybe good to prioritize a bit.
https://vitepress.dev/guide/data-loading#typed-data-loaders

import { defineLoader } from 'vitepress'

export interface Data {
  // data type
}

declare const data: Data
export { data }

export default defineLoader({
  // type checked loader options
  watch: ['...'],
  async load(): Promise<Data> {
    // ...
  }
})