feat(extensions): expose ExtensionRegistryObserver events in Session (#25385)

* feat(extensions): expose ExtensionRegistryObserver events in Session

Extensions can be loaded and unloaded for various reasons. In some cases this can
occur by no means of the Electron programmer, such as in the case of chrome.runtime.reload().

In order to be able to manage state about extensions outside of Electron's APIs, events
reloaded to loading and unloaded are needed.

* docs(extensions): elaborate on extension-loaded/unloaded details

* fix: remove scoped extension registry observer

* docs: update extension-unloaded

docs/api/session.md

@@ -91,6 +91,40 @@ session.defaultSession.on('will-download', (event, item, webContents) => {
 })
 ```
 
+#### Event: 'extension-loaded'
+
+Returns:
+
+* `event` Event
+* `extension` [Extension](structures/extension.md)
+
+Emitted after an extension is loaded. This occurs whenever an extension is
+added to the "enabled" set of extensions. This includes:
+- Extensions being loaded from `Session.loadExtension`.
+- Extensions being reloaded:
+   * from a crash.
+   * if the extension requested it ([`chrome.runtime.reload()`](https://developer.chrome.com/extensions/runtime#method-reload)).
+
+#### Event: 'extension-unloaded'
+
+Returns:
+
+* `event` Event
+* `extension` [Extension](structures/extension.md)
+
+Emitted after an extension is unloaded. This occurs when
+`Session.removeExtension` is called.
+
+#### Event: 'extension-ready'
+
+Returns:
+
+* `event` Event
+* `extension` [Extension](structures/extension.md)
+
+Emitted after an extension is loaded and all necessary browser state is
+initialized to support the start of the extension's background page.
+
 #### Event: 'preconnect'
 
 Returns: