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/dumper.rs

@@ -120,6 +120,18 @@ impl Dumper {
                 self.indent -= 1;
             }
 
+            BoundKind::DefDestructure { pattern, value } => {
+                self.log("DefDestructure", node);
+                self.indent += 1;
+                self.write_indent();
+                self.output.push_str("Pattern:\n");
+                self.visit(pattern);
+                self.write_indent();
+                self.output.push_str("Value:\n");
+                self.visit(value);
+                self.indent -= 1;
+            }
+
             BoundKind::If {
                 cond,
                 then_br,