* 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
`powerMonitor.querySystemIdleState` and `powerMonitor.querySystemIdleTime` had async backing APIs in chromium (https://chromium-review.googlesource.com/c/chromium/src/+/1379183). However, that has changed in ch73. So, this PR deprecates the old async APIs and adds new sync APIs.
* feat(BrowserWindow): expose interface to query system idle state
* test(BrowserWindow): update test cases for querySystemIdle interface
* docs(BrowserWindow): add querySystemIdle interface documentation
* refactor(powerMonitor): move querySystemIdle into powerMonitor
* test(powerMonitor): split test cases for all platform
renamed a few occurances of "web page" to "renderer"
renamed a few files that had "browser" in their name to "main-process"
note that there are still many occurances of web page.
