[Parsing / Rendering] Forbidden Items

Issue

We may want to re-think our parsing strategy for forbidden items.
Our current behavior of parsing forbidden item is dropping it and creates a warning.

We may want to re-think about dropping forbidden item in parsed card.

1. round tripping not possible.
2. our error handling is mostly done at render time. for example, we raise render warning if an element has invalid optional property. we raise render error if an element has invalid value for required property. Since we have skipped the forbidden element, we lose opportunity to alert the host app.

The down side of the change is that renderers will be burdened with recognizing forbidden items.

No files, tests, or entry points are named. Start by locating the parser and renderer implementations and their forbidden-item warning paths; define whether forbidden items remain in the parsed card and how render-time warnings or errors should work. Done requires an agreed design plus coverage for round-tripping and host-app error reporting.