add app.isPackaged (#12656)

* add isPackaged method

* add false test case for app.isPackaged()

* document add.isPackaged()

* check isPackaged() without fs call

* convert to non-method property

* document app.isPackaged as a property

* update tests for app.isPackaged

* remove unused fs require

* clarify docs for isPackaged()

lib/browser/api/app.js

@@ -1,6 +1,7 @@
 'use strict'
 
 const bindings = process.atomBinding('app')
+const path = require('path')
 const {app, App} = bindings
 
 // Only one app object permitted.
@@ -57,6 +58,14 @@ Object.assign(app, {
   }
 })
 
+app.isPackaged = (() => {
+  const execFile = path.basename(process.execPath).toLowerCase()
+  if (process.platform === 'win32') {
+    return execFile !== 'electron.exe'
+  }
+  return execFile !== 'electron'
+})()
+
 if (process.platform === 'darwin') {
   app.dock = {
     bounce (type = 'informational') {