Add `Promise.withResolvers()` polyfill for pdf.js

It's needed for both the main page (which we generate using jinja) and
the worker script.

Closes: https://github.com/qutebrowser/qutebrowser/issues/8170

qutebrowser/browser/pdfjs.py

@@ -69,6 +69,21 @@ def generate_pdfjs_page(filename, url):
     return html
 
 
+def _generate_polyfills():
+    return """
+        if (typeof Promise.withResolvers === 'undefined') {
+            Promise.withResolvers = function () {
+                let resolve, reject
+                const promise = new Promise((res, rej) => {
+                    resolve = res
+                    reject = rej
+                })
+                return { promise, resolve, reject }
+            }
+        }
+    """
+
+
 def _generate_pdfjs_script(filename):
     """Generate the script that shows the pdf with pdf.js.
 
@@ -83,6 +98,8 @@ def _generate_pdfjs_script(filename):
     js_url = javascript.to_js(url.toString(urlutils.FormatOption.ENCODED))
 
     return jinja.js_environment.from_string("""
+        {{ polyfills }}
+
         document.addEventListener("DOMContentLoaded", function() {
             if (typeof window.PDFJS !== 'undefined') {
                 // v1.x
@@ -104,7 +121,7 @@ def _generate_pdfjs_script(filename):
                 });
             }
         });
-    """).render(url=js_url)
+    """).render(url=js_url, polyfills=_generate_polyfills())
 
 
 def get_pdfjs_res_and_path(path):
@@ -148,6 +165,14 @@ def get_pdfjs_res_and_path(path):
             log.misc.warning("OSError while reading PDF.js file: {}".format(e))
             raise PDFJSNotFound(path) from None
 
+    if path == "build/pdf.worker.mjs":
+        content = b"\n".join(
+            [
+                _generate_polyfills().encode("ascii"),
+                content,
+            ]
+        )
+
     return content, file_path