Remove interactive-code-block from the main repository so it can live in a dedicated space. See #473. (#475)

## What?

Removes the interactive-code-block project from the main WordPress
playground repository so it can live in a dedicated space.

As outlined in #473, WordPress Playground is re-focusing on [the
vision](#472)
and separating the framework from the applications.

Once `interactive-code-block` is committed into a separate repo, a link
will be added to this PR.

Author: adamziel