feat: 添加对单引号字符串的支持，更新词法分析器和相关测试用例

src/interpreter/interpreter.rs

@@ -750,6 +750,30 @@ mod tests {
         }
     }
 
+    #[test]
+    fn eval_single_quoted_string() {
+        match eval_expr("'hello'") {
+            Value::String(s) => assert_eq!(s, "hello"),
+            v => panic!("Expected String, got {:?}", v),
+        }
+    }
+
+    #[test]
+    fn eval_mixed_quotes_concat() {
+        match eval_expr("\"a\" + 'b'") {
+            Value::String(s) => assert_eq!(s, "ab"),
+            v => panic!("Expected String, got {:?}", v),
+        }
+    }
+
+    #[test]
+    fn eval_single_quote_containing_double() {
+        match eval_expr("'he\"llo'") {
+            Value::String(s) => assert_eq!(s, "he\"llo"),
+            v => panic!("Expected String, got {:?}", v),
+        }
+    }
+
     #[test]
     fn eval_string_plus_number() {
         match eval_expr("\"a\" + 42") {