[ESM] Convert the "wintersmith"-task to use import() syntax

gulpfile.js

@@ -2039,12 +2039,14 @@ function ghPagesPrepare() {
   ]);
 }
 
-gulp.task("wintersmith", function (done) {
-  const wintersmith = require("wintersmith");
+gulp.task("wintersmith", async function () {
+  const { default: wintersmith } = await import("wintersmith");
   const env = wintersmith("docs/config.json");
+
+  return new Promise((resolve, reject) => {
     env.build(GH_PAGES_DIR, function (error) {
       if (error) {
-      done(error);
+        reject(error);
         return;
       }
 
@@ -2055,7 +2057,8 @@ gulp.task("wintersmith", function (done) {
       );
 
       console.log("Done building with wintersmith.");
-    done();
+      resolve();
+    });
   });
 });