Disallow identifiers starting with underscores

Modified lexer logic to reject identifiers starting with an underscore as they
are not supported by the default parser. Updated tests to reflect the new
validation and ensure proper error handling.

Author: mkpro118

src/core/scanner/lexer.rs

@@ -918,7 +918,7 @@ impl IdentifierRecognizer {
 impl TokenRecognizer for IdentifierRecognizer {
     fn can_handle(&self, input: &dyn CharacterStream) -> bool {
         if let Some(ch) = input.current() {
-            ch.is_alphabetic() || ch == '_'
+            ch.is_alphabetic()
         } else {
             false
         }
@@ -944,6 +944,19 @@ impl TokenRecognizer for IdentifierRecognizer {
                     span,
                 ));
             }
+
+            if identifier.is_empty() && ch == '_' {
+                let pos = input.position().to_symbol_location();
+                let span = SymbolSpan {
+                    start: pos.clone(),
+                    end: pos,
+                };
+                return Err(LexError::new(
+                    "Default parser does not support identifiers starting with underscore".to_owned(),
+                    span,
+                ));
+            }
+
             if ch.is_alphanumeric() || ch == '_' {
                 identifier.push(ch);
                 input.advance();
@@ -1575,8 +1588,6 @@ mod tests {
                 "with123numbers",
                 TokenType::Identifier("with123numbers".to_string()),
             ),
-            ("_", TokenType::Identifier("_".to_string())),
-            ("__private", TokenType::Identifier("__private".to_string())),
             ("true", TokenType::Literal("true".to_string())),
             ("false", TokenType::Literal("false".to_string())),
         ];
@@ -1586,6 +1597,14 @@ mod tests {
             let token = lexer.next_token().unwrap().unwrap();
             assert_eq!(*token.r#type(), expected, "Failed for input: {input}");
         }
+
+        let test_cases = vec!["_", "__private"];
+
+        for input in test_cases {
+            let mut lexer = Lexer::default_for_input(input);
+            let token = lexer.next_token();
+            assert!(token.is_err());
+        }
     }
 
     #[test]