Move to namespace allows a user to move all symbols in a context
to a different namespace, whether it's a new one or existing. This is
different from renaming a namespace as not all symbols in the namespace
are moved, only ones within the current document.

It relies on the IChangeNamespaceService implementation to do the heavy
lifting. Since the current implementation only supports C#, this new
feature will as well.

After this is checked in, when a user has the cursor inside a namespace
declaration that has no nested namespace declarations within it, a code
fix will be suggested to pop up a dialog and help the user choose a new
namespace.