support for folding code block when working on large livebooks

[epinault]

When using livebook it s really good but then when code block are large, it becomes sometime a long page to scroll down. Would be nice to be able to fold code block that I am not needing to touch while working on the others

Also being and to quick fold all code block too would be great! This would make the livebook quickly useful for just seeing the rendering quickly

I alsow wonder if sometime having the side by side frame between code and rendering as an option would be good