Refactor Handler default processor list

What does this MR do and why?

After adding a new processor in !2359 (merged) it was noted that the step to add the processor to the list of defaults in Triage::Handler had been missed. As a consequence the new processor will not run.

Although the instructions in the documentation are quite clear on this step, it's easily missed if creating a processor by cut-n-paste from an existing processor.

This MR is a proposal for a refactor to hopefully reduce the possibility of this happening in the future.