proposal-async-context: Async Context for JavaScript

The TC39 AsyncContext proposal (Stage 2) introduces a native JavaScript API for implicitly propagating values across asynchronous execution boundaries. It solves the long-standing problem of losing implicit call-site context when code crosses the event loop — something async/await makes nearly invisible. Two core APIs are proposed: AsyncContext.Variable for scoped value propagation, and AsyncContext.Snapshot for capturing and restoring context in scheduled callbacks.