perf(turbopack): Make `File` => json faster (#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"
/>
