Add Support for Postman 2.1 Export (most but not all elements/cases)

[Vyoam]

Closes #1156 (it was actually converted to a discussion, but this PR closes it properly)
Have discussed a bit with @dimitropoulos: Vyoam/InsomniaToPostmanFormat#4
Let's get this done. :)

Main notes:

• Adds support for Postman 2.1 export; mostly based on https://github.com/Vyoam/InsomniaToPostmanFormat
• Limitations listed here: [Improvement] Export collections using Postman format #3703 (comment)
• See the individual files in the PR for specific notes
• Verified import of the output json with Postman 8.8.0
• Started on May 29; worked on and off; completed around Jul 18; Develop branch changes were merged on Jul 18
• At one point, I did wonder if future changes in Insomnia's data structure will affect this export code. It might, but...
  1. this Postman export code simply uses the same object used for Insomnia's self export.
  2. any breaking change be easy to detect via the added UT.
  3. I guess har export code would anyway be updated in that situation, and this one can be done mostly along the same lines.

[Vyoam]

Marking self-review as finished.

[Vyoam]

As I don't have access to request reviews, tagging a few folks I found in the merged PRs.
Just want to have this closed and let everyone start using it. :)
@dimitropoulos
@develohpanda
@reynolek
@wdawson
@DMarby

[develohpanda]

Hi @Vyoam! Thanks for this PR. I saw the draft PR a few days ago and kicked off some discussions internally and we feel this would be better suited as a standalone plugin rather than bundled with the app. You can maintain, release and update plugins on your own accord via NPM, and they will show on the plugin hub for users to install. There is some documentation here for creating plugins - specifically, the data context (though this is slightly out of date so I encourage you to double check with the plugin contexts' as implemented as well).

You can generate a basic plugin from Preferences > Plugins, and feel free to reach out if you need any help or advice when creating a plugin.

[Vyoam]

Thanks @develohpanda. I understand. Will try the plugin approach. Closing this.