Refactor: Replace UntypedNode with SyntaxNode

This commit replaces the `UntypedNode` enum with the more accurately
named `SyntaxNode`. This change is primarily for clarity and better
reflects the role of these nodes as representing the structure of the
source code prior to semantic analysis.

The corresponding enum `UntypedKind` has also been renamed to
`SyntaxKind` to maintain consistency.

No functional changes are introduced by this refactoring; it is purely a
renaming and organizational update.

src/ast/compiler/mod.rs

@@ -1,21 +1,21 @@
-pub mod analyzer;
-pub mod binder;
-pub mod bound_nodes;
-pub mod captures;
-pub mod dumper;
-pub mod lambda_collector;
-pub mod macros;
-pub mod optimizer;
-pub mod specializer;
-pub mod lowering;
-pub mod type_checker;
-
-pub use binder::*;
-pub use bound_nodes::*;
-pub use captures::*;
-pub use dumper::*;
-pub use macros::*;
-pub use optimizer::*;
-pub use specializer::*;
-pub use lowering::*;
-pub use type_checker::*;
+pub mod analyzer;
+pub mod binder;
+pub mod bound_nodes;
+pub mod captures;
+pub mod dumper;
+pub mod lambda_collector;
+pub mod macros;
+pub mod optimizer;
+pub mod specializer;
+pub mod lowering;
+pub mod type_checker;
+
+pub use binder::*;
+pub use bound_nodes::*;
+pub use captures::*;
+pub use dumper::*;
+pub use macros::*;
+pub use optimizer::*;
+pub use specializer::*;
+pub use lowering::*;
+pub use type_checker::*;