[IMPROVE] Rewrite gulpfile in typescript

[aditya-mitra]

What? ⛵

Closes #393

1. Added:
   -gulpfile.ts,

• npm type definitions for gulp, gulp-bump and gulp-sourcemaps
• gupfile.d.ts for those modules which did not have type definitions found in npm

2. Updated:

• replaced CommonJS require with ES6 import statements in gulpfile.ts
• correction of buffer to be converted to string while doing JSON.parse

3. Removed:

• gulpfile.js

Why? 🤔

Since most of the codebase of Rocket.Chat.Apps-engine is written in typescript, it would be nicer if we could write gulpfile also in typescript. smiley

Links 🌎

Issue #393

PS 👀

[CLAassistant]

All committers have signed the CLA.

[aditya-mitra]

Hi,
The compilation is passing locally but failing on CircleCI. (I even tried this on a fresh GitPod instance and it was working)

What is the reason for this problem?