Use actual namespace for kscript annotations

[holgerbrandl]

Currently, kscript annotations live in the default package, so that users don't need to import them when writing scripts (which typically don't declare a package).

But the default package has issues and is considered bad practice https://youtrack.jetbrains.com/issue/KT-21422

So we may want to simply reimplement the annotations in an actual namespace (kscript.annotations) for better design and tooling within IJ.

[holgerbrandl]

Won't fix for now, but let's keep it open for discussion.

So far the only usecase that would benefit from an actual namespace are scripts that live in packages themselfes. See #81 for an example

[holgerbrandl]

Closed, since it is unlikely that we can resolve it anytime. It's a design flaw in favor of a more streamlined usage (less typing).