docs: update Route Handlers to have JS/TS code snippet. (#51003)

Added a TS and JS switcher. Previously the code snippet did not have a
switcher. The file extension was .js when the code was TypeScript which
made it confusing to try to read.

---------

Co-authored-by: Lee Robinson <me@leerob.io>

docs/02-app/01-building-your-application/01-routing/10-router-handlers.mdx

@@ -365,19 +365,21 @@ export async function GET(request) {
 
 Route Handlers can use [Dynamic Segments](/docs/app/building-your-application/routing/dynamic-routes) to create request handlers from dynamic data.
 
-```ts filename="app/items/[slug]/route.js"
+```ts filename="app/items/[slug]/route.ts" switcher
 export async function GET(
   request: Request,
-  {
-    params,
-  }: {
-    params: { slug: string }
-  }
+  { params }: { params: { slug: string } }
 ) {
   const slug = params.slug // 'a', 'b', or 'c'
 }
 ```
 
+```js filename="app/items/[slug]/route.js" switcher
+export async function GET(request, { params }) {
+  const slug = params.slug // 'a', 'b', or 'c'
+}
+```
+
 | Route                       | Example URL | `params`        |
 | --------------------------- | ----------- | --------------- |
 | `app/items/[slug]/route.js` | `/items/a`  | `{ slug: 'a' }` |