Parse a JSON dom out of an argument to a JSON function. @param[in] res Pointer to string value of arg. @param[in] arg_idx 0-based index of corresponding JSON function argument @param[in] func_name Name of the user-invoked JSON_ function @param[in,out] dom If non-null, we want any text parsed DOM returned at the location pointed to @param[in] require_str_or_json
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