Bulk Collection Import Support

[RyanObray]

I'm currently migrating from Postman and I have many collections. Something I'm finding quite painful is not being able to select multiple Postman exports at a time, forcing me to import one collection at a time. It would be very helpful for the import file selector to allow multi-select and for the import to process all collections selected at once.

[mikeedjones]

For openAPI specs I've put together #1685 - think the same approach would work for postman