Add 'again' macro for recursive expansion

The 'again' macro is a new addition to the macro system, allowing for
recursive macro expansion. This commit introduces the necessary AST
nodes, macro expansion logic, and an integration test to ensure its
functionality. The `soa_series.myc` example has also been updated to
demonstrate its usage.

src/ast/compiler/macros.rs

@@ -275,6 +275,17 @@ impl<E: MacroEvaluator> MacroExpander<E> {
                 })
             }
 
+            UntypedKind::Again { args } => {
+                let expanded_args = self.expand_recursive(*args)?;
+                Ok(Node {
+                    identity: node.identity,
+                    kind: UntypedKind::Again {
+                        args: Box::new(expanded_args),
+                    },
+                    ty: (),
+                })
+            }
+
             _ => Ok(node),
         }
     }
@@ -536,6 +547,17 @@ impl<E: MacroEvaluator> MacroExpander<E> {
                 })
             }
 
+            UntypedKind::Again { args } => {
+                let expanded_args = self.expand_template(*args, state)?;
+                Ok(Node {
+                    identity: node.identity,
+                    kind: UntypedKind::Again {
+                        args: Box::new(expanded_args),
+                    },
+                    ty: (),
+                })
+            }
+
             _ => Ok(node),
         }
     }