Badge request: SDK constraint and dependency version for Dart packages

The support for [Dart](https://dart.dev) packages is currently pretty basic: we can only get a bagde for the latest version of a package (i.e. `https://img.shields.io/pub/v/.svg`).

It exists a specification for the API endpoints exposed by [Pub](https://pub.dev) package manager:
https://github.com/dart-lang/pub/blob/master/doc/repository-spec-v2.md

The API let us access two missing things from `shields.io`:

- the SDK constraint: we can get the Node.js constraint from a npm package, the PHP constraint from a Packagist package, but it's currently impossible with a Dart package.
- the version of a dependency: same here. The badges are available for npm and Packagist packages but not for Pub packages.

It would be very cool if more kind of badges could be generated for Dart projets.

Start with Pub's repository API specification and compare the existing npm and Packagist badge implementations. Done means Shields can generate Dart package badges for the SDK constraint and for dependency versions.

Written by the indexing model from the issue text.