perf(turbopack): Make File => json faster (vercel#81279)

### What?

Considering the way `serde_json` works, I expect this to be faster.
Also, a JSON string is very likely to be a single `bytes::Bytes` already, so it would not regress the memory usage.

### Why?

<img width="843" alt="image"
src="https://github.com/user-attachments/assets/2c5ebb0b-335c-4ca1-aebd-64a6da9e0671"
/>

Author: kdy1

turbopack/crates/turbo-tasks-fs/src/lib.rs

@@ -1912,7 +1912,8 @@ impl FileContent {
     pub fn parse_json_ref(&self) -> FileJsonContent {
         match self {
             FileContent::Content(file) => {
-                let de = &mut serde_json::Deserializer::from_reader(file.read());
+                let content = file.content.clone().into_bytes();
+                let de = &mut serde_json::Deserializer::from_slice(&content);
                 match serde_path_to_error::deserialize(de) {
                     Ok(data) => FileJsonContent::Content(data),
                     Err(e) => FileJsonContent::Unparsable(Box::new(