add id to Listing with number attribute for cross reference

Author: gaoyuan

packages/mdbook-trpl/src/listing/mod.rs

@@ -221,7 +221,13 @@ struct Listing {
 
 impl Listing {
     fn opening_html(&self) -> String {
-        let figure = String::from("<figure class=\"listing\">\n");
+        let id_attribute = if let Some(number) = self.number.as_ref() {
+            format!(" id=\"listing-{number}\"")
+        } else {
+            String::new()
+        };
+
+        let figure = format!("<figure class=\"listing\"{id_attribute}>\n");
 
         match self.file_name.as_ref() {
             Some(file_name) => format!(