Allow plugins to return more than one release

[felipecrs]

New feature motivation

I'm extending semantic-release-vsce to support publishing to not only the Visual Studio Marketplace but also to the Open VSX Registry through felipecrs/semantic-release-vsce#169.

However, the semantic-release expects the return of the publish step to be an object with properties name and url. Now, as the plugin would be publishing to 2 different locations, it would be desired to be able to return a list with the two releases objects.

New feature description

The semantic-release to allow plugins to return a list of releases instead of just one release.

New feature implementation

The semantic-release can validate if it is a normal object, or if it is a list, and then validate each element of it as if it was one of the previously expected object. The implementation should not bring any backwards incompatibility.

[felipecrs]

Is a PR for this feature welcome?