Extends the define-let-to-double-define refactoring rule to handle multiple bindings in let expressions and renames it to define-let-to-multi-define to better reflect its new capability.

Problem

Previously, the rule could only refactor let expressions with a single binding:

(define a
  (let ([b 1])
    (+ b 10)))

would become:

(define b 1)
(define a (+ b 10))

However, it couldn't handle multiple bindings like this:

(define a
  (let ([b 1] [c 2])
    (+ b c 10)))

This is a valuable refactoring pattern that appears in real-world Racket code, such as in DrRacket.

Solution

The rule now accepts multiple bindings and generates the corresponding multiple define statements:

(define b 1)
(define c 2)
(define a (+ b c 10))

Safety checks ensure the refactoring only applies when:

• None of the new bindings would shadow existing identifiers in the context
• Later bindings don't depend on earlier ones (preventing invalid let* semantics)
• Free identifiers in the surrounding context aren't affected

Changes

• Extended pattern matching to accept [nested-id:id nested-expr:expr] ...+ instead of single binding
• Added check to prevent introducing bindings that conflict with existing identifiers
• Added check to ensure bindings are independent (no sequential dependencies)
• Renamed rule from define-let-to-double-define to define-let-to-multi-define
• Updated description to reflect that multiple defines may be produced
• Added comprehensive test suite covering single bindings, multiple bindings, and edge cases

All existing tests pass, confirming backward compatibility with single-binding cases.

Fixes #623

Fixes #627

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