feat: 实现for循环语句的解析和执行逻辑，更新词法分析器以支持for关键字

src/lexer/lexer.rs

@@ -243,6 +243,7 @@ impl Lexer {
             "if" => TokenKind::If,
             "else" => TokenKind::Else,
             "while" => TokenKind::While,
+            "for" => TokenKind::For,
             "return" => TokenKind::Return,
             "true" => TokenKind::True,
             "false" => TokenKind::False,