Add check for trailing tokens

Ensure that a script does not contain any tokens after the main
expression. This prevents accidental trailing expressions and enforces a
clearer script structure.

src/ast/parser.rs

@@ -46,6 +46,10 @@ impl<'a> Parser<'a> {
         }
     }
 
+    pub fn at_eof(&self) -> bool {
+        matches!(self.current_token.kind, TokenKind::EOF)
+    }
+
     fn parse_atom(&mut self) -> Result<Node<UntypedKind>, String> {
         let token = self.advance()?;
         let identity = Rc::new(NodeIdentity { location: token.location });