Expose pending-job cancellation in sys bindings

[undy-aeon]

Issue # (if available)

Depends on quickjs-ng/quickjs#1745.

Description of changes

Expose JS_DiscardPendingJobs in the platform bindings so hosts can release queued jobs without executing callbacks or destroying the runtime.

Tests cover empty and repeated discard, callback suppression, argument finalization, and runtime reuse. They also verify that jobs enqueued by native finalizers remain pending and that the returned count covers only the original queue.

This remains a draft until the engine API merges and the QuickJS submodule advances. The committed submodule is unchanged.

Validation on the latest upstream baseline, with the dependent engine patch applied locally: all 231 tests and doc tests in cargo test --all --no-default-features --features full-async passed on macOS, along with cargo fmt --all -- --check and cargo clippy --all --all-targets --features full-async,bindgen -- -D warnings.

Checklist

• Added change to the changelog
• Created unit tests for my feature if needed