Merged PR 115321: Upgraded Gulp to v4 to fix build on Node v12

Buddy build passed:
https://powerbi.visualstudio.com/Embedded/_build/results?buildId=2266972&view=results

## Changes in the PR

1. Upgraded Gulp to v4
Refer similar [Commit f1d2e5c7](f1d2e5c79a?refName=refs%2Fheads%2Fmaster&path=%2Fgulpfile.js): Merged PR 52328: Update gulp to version 4.0 in PowerBI-Models
1. Upgraded related packages
1. Added package-lock.json to gitignore
1. Deleted package-lock.json

Related work items: #438249

.gitignore

@@ -5,4 +5,5 @@ dist
 docs
 tmp
 .publish
-npm-debug.log*
+npm-debug.log*
+package-lock.json

gulpfile.js

@@ -1,7 +1,8 @@
-var gulp = require('gulp-help')(require('gulp'));
+var gulp = require('gulp');
 var del = require('del'),
   ghPages = require('gulp-gh-pages'),
   header = require('gulp-header'),
+  help = require('gulp-help-four'),
   fs = require('fs'),
   moment = require('moment'),
   rename = require('gulp-rename'),
@@ -14,10 +15,12 @@ var del = require('del'),
   webpackStream = require('webpack-stream'),
   webpackConfig = require('./webpack.config'),
   webpackTestConfig = require('./webpack.test.config'),
-  runSequence = require('run-sequence'),
+  runSequence = require('gulp4-run-sequence'),
   argv = require('yargs').argv
   ;
 
+help(gulp, undefined);
+
 var package = require('./package.json');
 var webpackBanner = package.name + " v" + package.version + " | (c) 2016 Microsoft Corporation " + package.license;
 var gulpBanner = "/*! " + webpackBanner + " */\n";

package.json

@@ -33,15 +33,16 @@
   },
   "devDependencies": {
     "del": "^2.2.1",
-    "gulp": "^3.9.1",
+    "gulp": "^4.0.2",
     "gulp-gh-pages": "^0.5.4",
     "gulp-header": "^1.8.7",
-    "gulp-help": "^1.6.1",
+    "gulp-help-four": "^0.2.3",
     "gulp-rename": "^1.2.2",
     "gulp-replace": "^0.5.4",
     "gulp-tslint": "^6.1.1",
     "gulp-typedoc": "^2.0.0",
     "gulp-uglify": "^1.5.3",
+    "gulp4-run-sequence": "^1.0.1",
     "jasmine": "^2.4.1",
     "jasmine-core": "^2.4.1",
     "jquery": "^2.2.3",
@@ -53,7 +54,6 @@
     "karma-spec-reporter": "0.0.26",
     "moment": "^2.14.1",
     "phantomjs-prebuilt": "^2.1.7",
-    "run-sequence": "^1.1.5",
     "ts-loader": "^0.8.2",
     "tslint": "^3.15.0",
     "typedoc": "^0.4.4",