Update series syntax and binder scope handling

Refine the BNF for `series` to explicitly include the `lookback_limit`
and `schema`.
Introduce scope management within the `bind` function for `if/else`
branches to ensure correct context handling during compilation.
Add `Again` and `GetField` node kinds to the `Specializer`.
Improve lexer to ignore invisible characters within identifiers,
demonstrated by a new test case.

src/ast/compiler/specializer.rs

@@ -143,6 +143,14 @@ impl Specializer {
                     node.ty.clone(),
                 )
             }
+            NodeKind::Again { args } => {
+                let args = Rc::new(self.visit_node(args.as_ref().clone()));
+                (NodeKind::Again { args }, node.ty.clone())
+            }
+            NodeKind::GetField { rec, field } => {
+                let rec = Rc::new(self.visit_node(rec.as_ref().clone()));
+                (NodeKind::GetField { rec, field }, node.ty.clone())
+            }
             k => (k, node.ty.clone()),
         };