Refactor UntypedKind::Parameter to Identifier

The `Parameter` kind in `UntypedKind` was only used for identifiers that
were being declared or referenced. This commit renames it to
`Identifier` and updates all the necessary code to reflect this change.
This simplifies the AST and makes it more consistent.

Additionally, a new macro `repeat` has been added to
`src/ast/system.myc`.

src/ast/nodes.rs

@@ -63,8 +63,8 @@ impl Clone for Box<dyn CustomNode> {
 pub enum UntypedKind {
     Nop,
     Constant(Value),
+    /// A general identifier (used for both references and declarations in the untyped AST).
     Identifier(Symbol),
-    Parameter(Symbol),
     /// A first-class field accessor (e.g. .name)
     FieldAccessor(crate::ast::types::Keyword),
     If {