Convert PDFPageProxy.prototype.getTextContent to an asynchronous method

This is a tiny bit shorter, which cannot hurt.

src/display/api.js

@@ -1739,7 +1739,7 @@ class PDFPageProxy {
    * @returns {Promise<TextContent>} A promise that is resolved with a
    *   {@link TextContent} object that represents the page's text content.
    */
-  getTextContent(params = {}) {
+  async getTextContent(params = {}) {
     if (this._transport._htmlForXfa) {
       // TODO: We need to revisit this once the XFA foreground patch lands and
       // only do this for non-foreground XFA.
@@ -1747,28 +1747,18 @@ class PDFPageProxy {
     }
     const readableStream = this.streamTextContent(params);
 
-    return new Promise(function (resolve, reject) {
-      function pump() {
-        reader.read().then(function ({ value, done }) {
-          if (done) {
-            resolve(textContent);
-            return;
-          }
-          textContent.lang ??= value.lang;
-          Object.assign(textContent.styles, value.styles);
-          textContent.items.push(...value.items);
-          pump();
-        }, reject);
-      }
+    const textContent = {
+      items: [],
+      styles: Object.create(null),
+      lang: null,
+    };
 
-      const reader = readableStream.getReader();
-      const textContent = {
-        items: [],
-        styles: Object.create(null),
-        lang: null,
-      };
-      pump();
-    });
+    for await (const value of readableStream) {
+      textContent.lang ??= value.lang;
+      Object.assign(textContent.styles, value.styles);
+      textContent.items.push(...value.items);
+    }
+    return textContent;
   }
 
   /**