Extra repository hook via `extra.repo.url` var

Previously defaulted to 'https://oss.sonatype.org/content/repositories/releases',
now must be set manually. PR validation uses it to configure the repository
that has the artifacts published during validation.
More useful ant targets for faster pr validation
IDE needs actors/swing/continuations publish for validation.

As soon as they are also modularized, we can go back to
only building/publishing the core (lib/reflect/compiler).
Bump starr so we can build continuations plugin with it.