Add support for destructuring def

This commit introduces the `DefDestructure` bound kind and modifies the
binder, analyzer, type checker, and VM to support destructuring in `def`
statements. This allows for pattern matching on the right-hand side of a
`def` to bind multiple variables.

The parser has been updated to accept patterns in `def` statements. The
binder now handles `UntypedKind::Def` with a `target` pattern, rather
than a simple `name`. This enables destructuring.

The `Gemini.md` documentation has been updated to include a new rule for
incremental development.

src/ast/nodes.rs

@@ -71,7 +71,7 @@ pub enum UntypedKind {
         else_br: Option<Box<Node<UntypedKind>>>,
     },
     Def {
-        name: Symbol,
+        target: Box<Node<UntypedKind>>,
         value: Box<Node<UntypedKind>>,
     },
     Assign {