feat: add json api end-point with versions list

[cmengler]

I'm using melos to manage a multi-package repository -- as part of the melos publish process, it retrieves a list of all versions from https://pub.dev/packages/[package].json end-point.

This PR adds a new API route /packages/[package].json that returns a JSON payload consisting of name and a list of versions strings -- e.g. https://pub.dev/packages/unpub.json

{
  "name": "unpub",
  "versions": [
    "2.0.0",
    "1.2.2",
    "1.2.1",
    "1.2.0",
    "1.1.0",
    "1.0.0",
    "0.4.0",
    "0.3.0",
    "0.2.2",
    "0.2.1",
    "0.2.0",
    "0.1.2",
    "0.1.1+1",
    "0.1.1",
    "0.1.0",
    "0.0.1"
  ]
}

[jrc2139]

worked perfectly for me. thanks!

[talisk]

LGTM. Thanks for your contribution.