Fixes a crash where plugins sending messages via 'penpot.ui.sendMessage()'
could fail if their message payload contained non-serializable values like
functions or closures.
The fix adds validation using 'structuredClone()' to catch these messages
early with a helpful error message, and adds a defensive try/catch in the
modal's message handler as a safety net.
Fixes the error: 'Failed to execute postMessage on Window: ... could not
be cloned.'
Signed-off-by: Andrey Antukh <niwi@niwi.nz>
