Added 'qualifier' back to core-im-source.yaml

[mbrush]

Larry dropped this in he latest refactor. But IMO defining qualifier in the core-im is important for two reasons. It is useful to explain what the notion of a ‘qualifier’ means and what it is used for in the modeling framework. And it is necessary to implement the conceptual specialization I describe above, which happens when a specific qualifier property is defined in a Statement profile.

in this PR:

• 'qualifier' is named using the singular name - consistent with it not taking an array in the core-im, and supporting the allowed 1:m specialization that is done when profiling this property.
• the property takes ‘object’ (which by my understanding covers any data type, including string, enums, IRIs, or nested/referenced structured objects)
• I gave it a clear and informative free-text description, and added a ‘comment’ that explains how this property can get profiled into >1 more specific qualifier properties

[larrybabb]

approving the placeholder comment to help us come to a resolution on how to deal with generating qualifiers in metaschema manager