Turbopack: add webpack loader names to traces (#87230)
When examining the long trace this morning, it wasn't immediately clear
what was running in the shell-out to Node.
This doesn't fully fix the issue, but it makes it clearer which webpack
loader is running, similar to how the PostCSS loader lists the name of
the css file it's parsing.
In this trace, the next config contains:
```
turbopack: {
rules: {
"**/*.{js,jsx,ts,tsx}": {
loaders: ["./loader_test.js"],
},
},
},
```
And the trace renders as:
<img width="995" height="553" alt="Screenshot 2025-12-15 at 11 22 39 PM"
src="https://github.com/user-attachments/assets/b0936e2e-f9d5-45fc-8a1c-2f98b1aa38b6"
/>
