Polyfill Array.prototype.at (#44436)

fixes issue #44141 , discussion #44148

This is missing and has caused me issues in production. Seems like a great polyfill to have, given that Next already polyfills so many adjacent Array methods.

## Bug

- [x] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [x] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)


Co-authored-by: Eyas Valdez <37156127+spiltbeans@users.noreply.github.com>
Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

packages/eslint-plugin-next/src/rules/no-unwanted-polyfillio.ts

@@ -3,6 +3,7 @@ import { defineRule } from '../utils/define-rule'
 // Keep in sync with next.js polyfills file : https://github.com/vercel/next.js/blob/master/packages/next-polyfill-nomodule/src/index.js
 const NEXT_POLYFILLED_FEATURES = [
   'Array.prototype.@@iterator',
+  'Array.prototype.at',
   'Array.prototype.copyWithin',
   'Array.prototype.fill',
   'Array.prototype.find',

packages/next-polyfill-module/src/index.js

@@ -118,3 +118,26 @@ if (!Object.fromEntries) {
     }, {})
   }
 }
+
+/**
+ * Available in:
+ * Internet Explorer: never
+ * Edge: 92
+ * Firefox: 90
+ * Chrome: 92
+ * Safari: 15.4
+ *
+ * https://caniuse.com/mdn-javascript_builtins_array_at
+ */
+// Modified from TC39 at proposal polyfill: https://github.com/tc39/proposal-relative-indexing-method#polyfill
+if (!Array.prototype.at) {
+  Array.prototype.at = function at(n) {
+    let i = Math.trunc(n) || 0
+
+    if (i < 0) i += this.length
+
+    if (i < 0 || i >= this.length) return undefined
+
+    return this[i]
+  }
+}

packages/next-polyfill-nomodule/src/index.js

@@ -1,5 +1,6 @@
 /* eslint-disable import/no-extraneous-dependencies */
 // Keep in sync with eslint no-unwanted-polyfillio rule: https://github.com/vercel/next.js/blob/master/packages/eslint-plugin-next/lib/rules/no-unwanted-polyfillio.js
+import 'core-js/features/array/at'
 import 'core-js/features/array/copy-within'
 import 'core-js/features/array/fill'
 import 'core-js/features/array/find'