[Discussion] `parse`, `toPlainJson`, `toPlainArray`, `toPlainMap`, `toPlainSet` should not return `undefined`

Returning `undefined` instead of throwing an error is an old approach and quite annoying because it requires a `!== undefined` check or non-null assertion on every call.

I would like these methods to throw instead of returning `undefined`. 

This will **not** change that, for example, array elements that fail to deserialize get substituted with `undefined`.