[Vue] Update rollup config

[michaeltintiuc]

Fixes rollup warnings:

(!) Mixing named and default exports
Consumers of your bundle will have to use bundle['default'] to access the default export, which may not be what you want. Use `output.exports: 'named'` to disable this warning

(!) Unresolved dependencies
https://github.com/rollup/rollup/wiki/Troubleshooting#treating-module-as-external-dependency
@ionic/core (imported by src/controllers/vue-delegate.ts)