Make PrefixedRouterContentSourceVc::new public (vercel/turborepo#5226)

### Description

It needs to be public for next-dev to instantiate it. 🤦 

### Testing Instructions

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crates/turbopack-dev-server/src/source/router.rs

@@ -28,7 +28,7 @@ pub struct PrefixedRouterContentSource {
 #[turbo_tasks::value_impl]
 impl PrefixedRouterContentSourceVc {
     #[turbo_tasks::function]
-    async fn new(
+    pub async fn new(
         prefix: StringVc,
         routes: Vec<(String, ContentSourceVc)>,
         fallback: ContentSourceVc,