...and a host object property on InteractiveScriptGlobals.

This is a first cut and my main goal at this point is forward
compatibility.  The existing unit tests all pass.

TODO: Make some improvements to the existing behavior.
TODO: Update the existing test baselines.
TODO: Introduce new tests.

This branch should be buddy testable - tweak the existing formatter by
subtyping (e.g.) `CSharpObjectFormatter` and assigning an instance of your
subtype to the global `ObjectFormatter` property.  It should be consumed
by subsequent submissions.