Workers - Event and EventTarget spec compliance improvements

Several improvements bring the Workers runtime's Event, EventTarget, and related APIs closer to their web platform specifications.

isTrusted is now correct for user-constructed events

User-constructed event subclasses (new MessageEvent(...), new CustomEvent(...), new ErrorEvent(...), new CloseEvent(...)) now correctly report isTrusted as false. Previously, these incorrectly reported isTrusted as true.

on<type> handlers fire in registration order

Event handler attributes such as onmessage and onabort now fire in their registration order relative to addEventListener() calls, matching browser behavior. Previously, on<type> handlers always fired before any addEventListener() listeners regardless of when they were assigned.

MessageEvent constructor improvements

new MessageEvent(type) now works without providing an init object — data defaults to null per spec. The MessageEventInit dictionary also now supports the full spec surface: origin, lastEventId, source, and ports, in addition to data.

CloseEvent and ErrorEvent constructor improvements

The CloseEventInit and ErrorEventInit dictionaries now accept the standard EventInit members bubbles, cancelable, and composed.

MessagePort starts on any message listener

Adding a message event listener to a MessagePort via addEventListener("message", ...) now starts the port, matching browser behavior. Previously, only assigning onmessage would start the port.