Standardise on has('debug') for debugging/testing features

[kitsonk]

When we need certain debugging features (or expose functionality for testing) we should standardise on this under the has('debug'); feature.

[dylans]

If we're going to do this, it should probably be a beta blocker.

[kitsonk]

We should use this ticket to identify structures, there are already a few. With the static build optimisation, CLI already has a concept of "debug" builds, which we can continue to leverage.