Add back() and forward() to new router (vercel#41575)

Adds `router.back()` and `router.forward()`.
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## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the
feature request has been accepted for implementation before opening a
PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration 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`

## Documentation / Examples

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

Author: timneutkens

packages/next/client/components/app-router.tsx

@@ -203,6 +203,8 @@ function Router({
     }
 
     const routerInstance: AppRouterInstance = {
+      back: () => window.history.back(),
+      forward: () => window.history.forward(),
       // TODO-APP: implement prefetching of flight
       prefetch: async (href) => {
         // If prefetch has already been triggered, don't trigger it again.

packages/next/shared/lib/app-router-context.ts

@@ -29,6 +29,14 @@ interface NavigateOptions {
 }
 
 export interface AppRouterInstance {
+  /**
+   * Navigate to the previous history entry.
+   */
+  back(): void
+  /**
+   * Navigate to the next history entry.
+   */
+  forward(): void
   /**
    * Refresh the current page.
    */