Generic function for an explicit, on-demand preview of an object, as
distinct from print(). preview() exists for object types
where print() is shared with another package's S3 generic
dispatch (e.g. class "html", used both by pharos and
htmltools for genuinely different purposes), so that registering
an own print.* method would silently overwrite - or be
overwritten by - the other package's behaviour.
Details
The default method simply calls print(), so preview() is
always safe to call even for types with no dedicated method.
