Change list comprehensions' body from ScalarExpr to Expr

The current implementation for nested list-comprehensions only supports creating Vectors of ScalarExpressions, no matter the nesting level

The goal here would be to enable a more general form of nested list comprehensions, akin to a Matrix comprehension.

Based off of this PR:
https://github.com/0xMiden/air-script/pull/400#issue-3065707667

Task list:
- [ ] Replace list comprehension bodies from ScalarExpr to Expr
- [ ] Ensure all dimension transformations fit the new general form, I think there may be a few cases where they are not handled in a compatible way due to design assumptions
- [ ] update de docs