Support directly generating Postman Collections JSON

It would be useful to be able to export directly to Postman collections format (v2 - structure documented [here](https://schema.postman.com/collection/json/v2.1.0/draft-07/docs/index.html)).

There are other existing JS libraries to do this (e.g. https://github.com/javieraviles/Har2Postman/) and it's very simple (it's a JSON -> JSON transformation) but integrating that in Httpsnippet too would be useful, to make it available within the same structure & using the same API as all the other HTTP export formats here.

Start by reviewing Httpsnippet's existing HTTP export format implementations and the shared API used by them, then compare the requested output with the Postman Collections v2.1 schema and the linked Har2Postman library. Done means direct generation of valid Postman Collections JSON through the same API as the other formats.

索引モデルが issue の本文から書いたものです。