Some tests need real OS threading and timers rather than the
deterministic, single-threaded behavior of `TestDispatcher`. The
existing multithreaded dispatcher provides those production-like
scheduling semantics, but it is named and gated as a benchmark-only
facility, which unnecessarily pulls in Criterion and histogram
dependencies.

This renames `BenchDispatcher` to `ThreadedDispatcher` and exposes it
through GPUI's `test-support` feature. Benchmark contexts continue using
the same dispatcher, while Criterion and benchmark reporting remain
isolated behind the `bench` feature.

### Testing

- `cargo check -p gpui --features test-support`
- `cargo check -p gpui --features bench`
- `cargo nextest run -p gpui 'threaded_dispatcher::tests'`
- `cargo fmt --all --check`

Release Notes:

- N/A