Add JoinHandle::abort()

Editor assessment

The subject names the new method exactly: JoinHandle::abort(). Type feat is accurate, the full path in backticks makes the change verifiable, and the missing scope is a small gap where core would fit, though the type name anchors it. No body, acceptable for a self-evident task-cancellation method. The Co-authored-by footer is well formed. No hygiene problems beyond the PR number. A tight, complete message for a small, expected API addition.