Add configuration option for adding additional Node.js dependencies
Currently, the plugin only installs the Antora core platform. If additional dependencies are needed, the user has to create a package.json file and list them there. However, that's at odds with the goal of this plugin to encapsulate the configuration in the Gradle build (in other words, it's a leaky abstraction). It should be possible to specify additional Node.js dependencies in the antora
configuration block that get installed alongside Antora. These dependencies are usually Antora extensions that the site depends on.
The format for this configuration option should be a map of package name and version/location. Ultimately, these entries end up being mapped to the dependencies
key in package.json (or installed as though they were listed there).