Support multi-valued comprehensions (EEP-78)

[sile]

This PR implements support for multi-valued comprehensions as specified in EEP-78, allowing multiple elements to be emitted per iteration in list and map comprehensions.

Changes

• components.rs: Add children() method for ComprehensionExpr<Open, Close, NonEmptyItems<Expr>> to iterate over all expression and qualifier children
• lists.rs:
  • Update ListComprehensionExpr to use NonEmptyItems<Expr> to support multiple value expressions
  • Update documentation to reflect the new syntax [Expr, ...]
  • Add test cases for multi-valued list comprehensions
• maps.rs:
  • Update MapComprehensionExpr to use NonEmptyItems<MapComprehensionValue> for multiple entries per iteration
  • Implement children() method for map comprehensions that flattens multiple key-value pairs
  • Update documentation and add test cases for multi-valued map comprehensions

These changes enable syntax like [X, Y || X <- Xs, Y <- Ys] and #{K1 => V1, K2 => V2 || ...} while maintaining backwards compatibility with single-valued comprehensions.