Add hierarchical evaluation cancellation to Boa

Normalized profile

What this task asks for

Add cancellable evaluation handles that propagate through nested script, module, and job execution. `Context::{eval_with_evaluation, enqueue_job_with_evaluation, run_jobs_with_evaluation}` must each return a fallible result with the same result-shape category as its non-handle analog. `cancellation_reason(context)` must return an optional value (`None` when not cancelled, `Some(reason)` when cancelled). For an already-cancelled handle, `Module::evaluate_with_evaluation` must still return success with a rejected promise. `Context::enqueue_job_with_evaluation(job, handle)` must fail immediately when `handle` is already cancelled and must not enqueue that job. If cancellation happens without a custom reason, `cancellation_reason(context)` must produce an Error-like value whose string contains `AbortError`. `Context::run_jobs_with_evaluation(handle)` must fail immediately when `handle` is already cancelled and must not drain queued jobs in that failed call.