feat: 添加对const关键字的支持，允许定义不可变变量并增强相关错误处理

src/lexer/lexer.rs

@@ -315,6 +315,7 @@ impl Lexer {
 
         let kind = match s.as_str() {
             "let" => TokenKind::Let,
+            "const" => TokenKind::Const,
             "fn" => TokenKind::Fn,
             "if" => TokenKind::If,
             "else" => TokenKind::Else,
@@ -532,9 +533,9 @@ mod tests {
 
     #[test]
     fn keywords() {
-        let kinds = tokenize("let fn if else while for break continue return true false nil");
+        let kinds = tokenize("let const fn if else while for break continue return true false nil");
         assert_eq!(kinds, vec![
-            TokenKind::Let, TokenKind::Fn,
+            TokenKind::Let, TokenKind::Const, TokenKind::Fn,
             TokenKind::If, TokenKind::Else,
             TokenKind::While, TokenKind::For,
             TokenKind::Break, TokenKind::Continue,