Use local version of gulp in build steps

fixes #3635

This reverts commit 69fa46da.

Fixes #1579

Previously developers had to install gulp globally on their machine
to build the project. These changes changes that by using the already
locally installed version of gulp, removing the environment requirements
aswell as ensuring everyone are using the same version of gulp.

It fixes the VS Code build tasks, aswell as creating a npm script to trigger the watch task:

```bash
$ npm run watch
```

Simplify test script definition in package.json

fixes #3589

As npm puts files from `./node_modules/.bin` in `$PATH` when running npm scripts, running bin commands from npm dependencies could be run directly, without a bin script deep inside `node_modules`.

P.S. great editor you've made so far!