Blocks UI needs a way to do parallel list comprehensions

Parallel list comprehensions are actually a widely used feature by students in CodeWorld classes, and provide convenient ways to do a bunch of stuff, such as alternating colors or placing things in random spots (by shuffling the x and y coordinates with different seeds). Definitely worth adding to the blocks version of list comprehensions.

Start by locating the Blocks UI implementation for list comprehensions and reading how the current blocks representation handles them. Determine how parallel list comprehensions should be represented and what student-facing behavior counts as complete; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.