Proposal: Add a `type` field

[Azathothas]

Hi, Currently, there's no way to distinguish between library or executable (cli) Nim packages.

I propose a new field: type.

It would be an array and could contain:

1. If the package is an executable only package:

{
  "type": [
    "executable"
  ]
}

2. If the package is a library only package:

{
  "type": [
    "library"
  ]
}

3. If the package is both executable & library:

{
  "type": [
    "library",
    "executable"
  ]
}

This would enable better package discovery and filtering capabilities.