We should only send previews of ordinary objects[^1] (i think that's the
correct term) objects to the devtools. The code is not designed to
handle other objects, which currently causes crashes.
Testing: This change adds a devtools test that crashes without this
change.
[^1]: https://tc39.es/ecma262/#ordinary-object
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Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>