Add the notification delegate API surface for the browser.notifications web extension API

Component: WebKit Web Extensions | 0a43f59

WKWebExtensionController implements Safari's Web Extensions support, which lets extension JavaScript call a browser.* namespace API compatible with Chrome and Firefox extensions. Because WebKit itself has no native notification UI, features like this are implemented via an app delegate pattern: the extension's request crosses into the UIProcess, and the embedding app is asked to fulfill it using platform-native APIs, with the app also responsible for reporting back permission state. This commit adds the UIProcess-side Objective-C API surface for browser.notifications: a new _WKWebExtensionNotification model class and four optional WKWebExtensionControllerDelegate methods covering present, update, and clear notification, plus a query for permission level. Declarations only — no implementation yet.

Significance

This establishes the app-facing contract that will let Safari Web Extensions trigger native notifications through the host app, ahead of Chrome/Firefox browser.notifications compatibility. Permission state is reported back through mayPresentNotificationsForExtensionContext, which places the authorization decision in the embedding app rather than in WebKit.

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