feat: 添加for-in循环支持，更新解析器、解释器和相关测试用例

src/lexer/lexer.rs

@@ -322,6 +322,7 @@ impl Lexer {
             "else" => TokenKind::Else,
             "while" => TokenKind::While,
             "for" => TokenKind::For,
+            "in" => TokenKind::In,
             "break" => TokenKind::Break,
             "continue" => TokenKind::Continue,
             "return" => TokenKind::Return,
@@ -534,11 +535,11 @@ mod tests {
 
     #[test]
     fn keywords() {
-        let kinds = tokenize("let const fn if else while for break continue return true false nil");
+        let kinds = tokenize("let const fn if else while for in break continue return true false nil");
         assert_eq!(kinds, vec![
             TokenKind::Let, TokenKind::Const, TokenKind::Fn,
             TokenKind::If, TokenKind::Else,
-            TokenKind::While, TokenKind::For,
+            TokenKind::While, TokenKind::For, TokenKind::In,
             TokenKind::Break, TokenKind::Continue,
             TokenKind::Return,
             TokenKind::True, TokenKind::False,