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/compiler/tco.rs

@@ -118,6 +118,10 @@ impl TCO {
                 global_index: *global_index,
                 value: Box::new(Self::transform(Rc::new((**value).clone()), false)),
             },
+            BoundKind::DefDestructure { pattern, value } => BoundKind::DefDestructure {
+                pattern: Box::new(Self::transform(Rc::new((**pattern).clone()), false)),
+                value: Box::new(Self::transform(Rc::new((**value).clone()), false)),
+            },
             BoundKind::Record { fields } => {
                 let new_fields = fields
                     .iter()