Add type to provider JSON

[stefaanneyts]

The provider JSON does not include type at the package level. Compose will fallback to the default type which is "library". Packagist.org has types like "project", "application", "composer-plugin", ... This fix will output the correct type in the provider JSON.

This pull request makes the following changes:

• Add type to provider JSON.

[sonatypecla[bot]]

Thanks for the contribution! Before we can merge this, we need @stefaanneyts to sign the Sonatype Contributor License Agreement.

[fjmilens3]

@stefaanneyts, thanks for putting this together! Before I merge, could you do the following?

• Add a corresponding test to the unit tests to cover the addition of the new field. Take a look at #30 to see where those changes would need to go as that PR is very similar (adding some missing fields, we started out with too restrictive a subset of fields, as you also noticed).
• Add your name to the CONTRIBUTORS.md file in the External contributors section. We appreciate all the contributions we get on this project, so we want to make sure you're mentioned accordingly.

[stefaanneyts]

@fjmilens3

• the corresponding tests are added to the unit tests
• my name is added to the CONTRIBUTORS.md