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/mod.rs

@@ -1,9 +1,9 @@
-pub mod compiler;
-pub mod diagnostics;
-pub mod environment;
-pub mod lexer;
-pub mod nodes;
-pub mod parser;
-pub mod rtl;
-pub mod types;
-pub mod vm;
+pub mod compiler;
+pub mod diagnostics;
+pub mod environment;
+pub mod lexer;
+pub mod nodes;
+pub mod parser;
+pub mod rtl;
+pub mod types;
+pub mod vm;