compat: polyfill AbortController/AbortSignal on older browsers (#2371)

apps/readest-app/src/utils/polyfill.ts

@@ -0,0 +1,31 @@
+import 'abortcontroller-polyfill/dist/abortcontroller-polyfill-only';
+
+// A groupBy polyfill for foliate-js
+Object.groupBy ??= (iterable, callbackfn) => {
+  const obj = Object.create(null);
+  let i = 0;
+  for (const value of iterable) {
+    const key = callbackfn(value, i++);
+    if (key in obj) {
+      obj[key].push(value);
+    } else {
+      obj[key] = [value];
+    }
+  }
+  return obj;
+};
+
+Map.groupBy ??= (iterable, callbackfn) => {
+  const map = new Map();
+  let i = 0;
+  for (const value of iterable) {
+    const key = callbackfn(value, i++),
+      list = map.get(key);
+    if (list) {
+      list.push(value);
+    } else {
+      map.set(key, [value]);
+    }
+  }
+  return map;
+};