Error on folded scalar in code mode

[ingydotnet]

YS errors on block sequences and flow collections when in "code mode". The reason is twofold:

• They aren't really valuable in code mode
• When you see them, you must be in data mode. Improves recognition when reading YS code.

Folded scalars arguably fall into the same are (although they aren't very useful in data mode either).

Goal: Error on folded scalars in code mode.