### Preflight Checklist
* [x] I have read the [Contributing Guidelines](https://github.com/electron/electron/blob/master/CONTRIBUTING.md) for this project.
* [x] I agree to follow the [Code of Conduct](https://github.com/electron/electron/blob/master/CODE_OF_CONDUCT.md) that this project adheres to.
* [x] I have searched the issue tracker for a feature request that matches the one I want to file, without success.

### Problem Description
When working with BrowserViews, there's no way to listen to focus changes on either BrowserView itself or its WebContents.

### Proposed Solution
On BrowserWindow, there are `focus` and `blur` events. The same events need to be emitted on BrowserView/WebContents.
i.e. : 
```
browserView.on('focus', () => {
    // view is focused, do stuff
});
```
or  
```
browserView.webContents.on('focus', () => {
    // contents are focused, do stuff
});
```

### Alternatives Considered
I can't think of a good way of working around this lack of functionality
shell/browser/api/electron_api_web_contents.cc
ontents()->NotifyPreferencesChanged();
  WebContentsPermissionHelper::CreateForWebContents(ontents());
  InitZoomController(ontents(), options);
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
  extensions::ElectronExtensionWebContentsObserver::CreateForWebContents(
      ontents());
  script_executor_ =
      std::make_unique<extensions::ScriptExecutor>(ontents());
#endif
  AutofillDriverFactory::CreateForWebContents(ontents());
  ontents()->SetUserAgentOverride(blink::UserAgentOverride::UserAgentOnly(
                                           GetBrowserContext()->GetUserAgent()),
                                       false);
  if (IsGuest()) {
    NativeWindow* owner_window = nullptr;
    if (embedder_) {
      
      auto* relay =
          NativeWindowRelay::FromWebContents(embedder_->ontents());
      if (relay)
        owner_window = relay->GetNativeWindow();
    }
    if (owner_window)
      SetOwnerWindow(owner_window);
  }
  ontents()->SetUserData(kElectronApiWebContentsKey,
                              std::make_unique<UserDataLink>(GetWeakPtr()));
}
#if BUILDFLAG(ENABLE_ELECTRON_EXTENSIONS)
void WebContents::InitWithExtensionView(v8::Isolate* isolate,