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/rtl/mod.rs

@@ -1,17 +1,17 @@
-pub mod core;
-pub mod datetime;
-pub mod intrinsics;
-pub mod math;
-pub mod series;
-pub mod streams;
-pub mod type_registry;
-
-use crate::ast::environment::Environment;
-
-pub fn register(env: &Environment) {
-    core::register(env);
-    datetime::register(env);
-    math::register(env);
-    series::register(env);
-    streams::register(env);
-}
+pub mod core;
+pub mod datetime;
+pub mod intrinsics;
+pub mod math;
+pub mod series;
+pub mod streams;
+pub mod type_registry;
+
+use crate::ast::environment::Environment;
+
+pub fn register(env: &Environment) {
+    core::register(env);
+    datetime::register(env);
+    math::register(env);
+    series::register(env);
+    streams::register(env);
+}