feat: Implement AST macros and templates

This commit introduces support for AST macros and templates, enabling
users to define and use custom, reusable components within the visual
DSL.

Key changes include:
- A new `MacroRegistry` to manage macro definitions.
- A `MacroExpander` to process macro calls and expand templates.
- A `MacroEvaluator` trait for evaluating expressions during expansion.
- The `Expansion` node in `BoundKind` to preserve the original macro
  call and its expanded form for debugging.
- Updates to the `Binder`, `Dumper`, and `UpvalueAnalyzer` to handle the
  new macro constructs.
- New examples demonstrating various macro functionalities like
  `unless`, splicing, and nested macros.

src/ast/compiler/binder.rs

@@ -300,6 +300,19 @@ impl Binder {
                 
                 Ok(self.make_node(node.identity.clone(), BoundKind::Map { entries: bound_entries }, ty))
             },
+
+            UntypedKind::Expansion { call, expanded } => {
+                let bound_expanded = self.bind(expanded)?;
+                let ty = bound_expanded.ty.clone();
+                Ok(self.make_node(node.identity.clone(), BoundKind::Expansion {
+                    original_call: Rc::from(call.as_ref().clone()),
+                    bound_expanded: Box::new(bound_expanded),
+                }, ty))
+            },
+
+            UntypedKind::Template(_) | UntypedKind::Placeholder(_) | UntypedKind::Splice(_) | UntypedKind::MacroDecl { .. } => {
+                Err(format!("Macro construct {:?} found in Binder. Macros must be expanded before binding.", node.kind))
+            }
             
             UntypedKind::Extension(_) => {
                 Err("Custom extensions not supported in Binder yet".to_string())