Publication validator

This is a supplementary subproject of kotlinx.coroutines that provides a new task, testPublishing, to test its publication correctness.

The tests are the following:

• NpmPublicationValidator tests that version of NPM artifact is correct and that it has neither source nor package dependencies on atomicfu
• MavenPublicationValidator depends on the published artifacts and tests artifacts binary content and absence of atomicfu in the classpath

To test publication, one needs to run gradle with -PdryRun=true, and the task that actually does the testing is publication-validator:test. -PdryRun affects npmPublish so that it only provides a packed publication and does not in fact attempt to send the build for publication.