Add "new plugin" templates

[dmikurube]

The Embulk core has had a feature to generate a "new Embulk plugin" from template: embulk new.

It was convenient, but as a result, the template has depended on "Embulk versions", and it was not very easy for core developers to maintain the "new Embulk plugin" feature in the Embulk core. For example, applying code format.

The "new Embulk plugin" template feature will be moved to this Gradle plugin so that the development will be easier.