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/parser/parser.rs

@@ -37,6 +37,8 @@ impl Parser {
             self.if_statement()
         } else if self.match_kind(&[TokenKind::While]) {
             self.while_statement()
+        } else if self.match_kind(&[TokenKind::For]) {
+            self.for_statement()
         } else if self.match_kind(&[TokenKind::Return]) {
             self.return_statement()
         } else if self.match_kind(&[TokenKind::LeftBrace]) {
@@ -117,6 +119,40 @@ impl Parser {
         Stmt::While { condition, body }
     }
 
+    fn for_statement(&mut self) -> Stmt {
+        self.consume(TokenKind::LeftParen, "Expected '(' after 'for'.").unwrap();
+        let initializer = if self.match_kind(&[TokenKind::Semicolon]) {
+            None
+        } else if self.match_kind(&[TokenKind::Let]) {
+            Some(self.let_declaration())
+        } else {
+            Some(self.expression_statement())
+        };
+
+        let condition = if !self.check(&TokenKind::Semicolon) {
+            Some(self.expression())
+        } else {
+            None
+        };
+
+        self.consume(TokenKind::Semicolon, "Expected ';' after loop condition.").unwrap();
+        let step = if !self.check(&TokenKind::RightParen) {
+            Some(self.expression())
+        } else {
+            None
+        };
+
+        self.consume(TokenKind::RightParen, "Expected ')' after for clauses.").unwrap();
+        let body = Box::new(self.statement());
+
+        Stmt::For {
+            initializer: initializer.map(Box::new),
+            condition,
+            step,
+            body
+        }
+    }
+
     fn return_statement(&mut self) -> Stmt {
         let value = if !self.check(&TokenKind::Semicolon) && !self.check(&TokenKind::RightBrace) {
             Some(self.expression())