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()

spec/api-app-spec.js

@@ -118,6 +118,12 @@ describe('app module', () => {
     })
   })
 
+  describe('app.isPackaged', () => {
+    it('should be false durings tests', () => {
+      assert.equal(app.isPackaged, false)
+    })
+  })
+
   describe('app.isInApplicationsFolder()', () => {
     before(function () {
       if (process.platform !== 'darwin') {