@vue/compat TS shim needs `export {}` to work properly

[snoozbuster]

In the migration build steps, the typescript shim that's provided only works if there is an import or export at the top level of the file it's placed in, otherwise it doesn't cause TS to see the export * from '@vue/runtime-dom'. There is a relevant sentence in the Vue 3 docs about type augmentation vs type overwriting, which appears to be what's happening here.