refactor: prefer using app.whenReady() (#21972)

* docs: add references to app.whenReady() in isReady

* refactor: prefer app.whenReady()

In the docs, specs, and lib, replace instances of `app.once('ready')`
(seen occasionally) and `app.on('ready')` (extremely common) with
`app.whenReady()`.

It's better to encourage users to use whenReady():
1. it handles the edge case of registering for 'ready' after it's fired
2. it avoids the minor wart of leaving an active listener alive for
an event that wll never fire again

docs/api/protocol.md

@@ -11,7 +11,7 @@ An example of implementing a protocol that has the same effect as the
 const { app, protocol } = require('electron')
 const path = require('path')
 
-app.on('ready', () => {
+app.whenReady().then(() => {
   protocol.registerFileProtocol('atom', (request, callback) => {
     const url = request.url.substr(7)
     callback({ path: path.normalize(`${__dirname}/${url}`) })
@@ -34,7 +34,7 @@ To have your custom protocol work in combination with a custom session, you need
 const { session, app, protocol } = require('electron')
 const path = require('path')
 
-app.on('ready', () => {
+app.whenReady().then(() => {
   const partition = 'persist:example'
   const ses = session.fromPartition(partition)