Implement folding support

[jvalkeal]

Relates to spring-attic/spring-dsl#21

[jvalkeal]

Moving to next release as this folding this is a bit pain to implement. It doesn't really break anything in UI but these inconsistent folds are a nuisance. It's consistent with a fully structured language but like when recursive blocks kinda start from a same like things fall apart.

We're only tried these things with stream language for now but there's same issue for other languages as well. Need to think about what is the actual folding generic structure.