Remove unused turbo-tasks items and add lint check (#90801)
### What?
Adds a lint script (`scripts/check-unused-turbo-tasks.mjs`) that detects
unused `#[turbo_tasks::function]`, `#[turbo_tasks::value]`, and
`#[turbo_tasks::value_trait]` definitions across the Rust codebase, and
removes all currently unused items.
### Why?
Dead turbo-tasks definitions add compilation overhead (each generates
trait impls, registrations, etc.) and make the codebase harder to
navigate. A lint check prevents regressions.
### How?
**New script** (`scripts/check-unused-turbo-tasks.mjs`):
1. Discovers all `.rs` files in `turbopack/crates/` and `crates/`
(excluding `turbo-tasks-macros-tests`)
2. Parses three kinds of turbo-tasks annotations:
- `#[turbo_tasks::function]` → extracts `fn` name + context (free,
inherent impl, trait impl, trait default)
- `#[turbo_tasks::value]` → extracts `struct`/`enum` name
- `#[turbo_tasks::value_trait]` → extracts `trait` name
3. Scans all files for identifier references, excluding definition sites
4. Reports any name that has no external references
The script is added to the `pnpm lint` command (runs in parallel with
other checks).
**Removed 23 unused items** across 18 files:
| Kind | Count | Examples |
|------|-------|---------|
| `value` (struct/enum) | 9 | `OutputAssetsWithAvailability`,
`ModuleNameMap`, `OptionModuleIds`, `LockedVersions`,
`AssetContentSnapshot` |
| `function` | 14 | `client_env`, `next_js_file`, `convert_path`,
`is_hot_module_replacement_enabled`,
`any_content_changed_of_output_asset` |
Also cleaned up imports that became unused after the removals.
