Turn the new expression language on by default

Problem

The issue Replace the expression language with Moa (#300) introduces the Moa expression language to the step-runner. As a precaution, and to allow for further testing, the expression language will be released under a feature flag that is off by default.

Proposal

Once testing is complete, default the feature flag to on. Remove the feature flag if confidence is high that the expression language is ready.

Update tests in the step-runner, taking note of any breaking changes that will need to be documented.