feat: use the postman-collections sdk

[nohehf]

Currently, the format.ts module is building the postman collection programmatically from scratch. However, Postman has an npm package: https://github.com/postmanlabs/postman-collection that provides an SDK for doing so.
I decided not to use it in the fireplace as deno support for npm packages was not a thing back then, but it has evolved a lot since then.
It would be interesting to use the SDK for cleaner code, reducing the code-base and future-proofing.

Edit: as of deno 1.26.x npm support is still unstable, I'll get onto this issue when I'm satisfied with this: denoland/deno#15960