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.
Move `Purity` enum definition from `optimizer.rs` to `types.rs` and
create a `NativeFunction` struct to hold the function and its purity.
Update `Value::Function` to store `Rc<NativeFunction>` and
`Value::make_function`
helper to simplify creation.
This change allows tracking the purity of native functions, which is
useful
for optimization and static analysis.