docs: correct documentation for buildscript block, add local confirmProjectVersionIncrementedTask

Small fix to correct the example provided in the README.

Also includes a way to invoke the confirmProjectVersionIncrementedTask, sharing the logic with the task in the release.

See merge request !1

We don't have a facility to apply the plugin itself without fetching an earlier version.
Includes a script that delegates to the same logic as the task.

Include 'confirmProjectVersionIncremented' task, tests, and basic docs.