ifcdiff: fix crash when exporting property diffs to JSON

DeepDiff's dictionary_item_added/set_item_added results are a
deepdiff.helper.SetOrdered instance, which subclasses orderly_set's
StableSetEq rather than the OrderedSet class json_dump_default checked
for, so the property relationship check always crashed export() with
"Object of type SetOrdered is not JSON serializable". Check against
StableSet, the common base class shared by every orderly_set set
flavour, instead.

Fixes #8905

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