Add MDXComponents type to useMDXComponents (#45737)

MDX publishes types which includes a proper `MDXComponents` type,
provides nicer autocomplete for users and also includes the `wrapper`
option:


![image](https://user-images.githubusercontent.com/55017335/217798272-2fc5e574-5a0b-49e5-92d8-62b56ad4d043.png)



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## Bug

- [ ] 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.
- [ ] 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

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

examples/app-dir-mdx/mdx-components.tsx

@@ -1,7 +1,7 @@
+import type { MDXComponents } from 'mdx/types'
+
 // This file is required to use MDX in `app` directory.
-export function useMDXComponents(components: {
-  [component: string]: React.ComponentType
-}) {
+export function useMDXComponents(components: MDXComponents): MDXComponents {
   return {
     // Allows customizing built-in components, e.g. to add styling.
     // h1: ({ children }) => <h1 style={{ fontSize: "100px" }}>{children}</h1>,