Remove dependency on `gulp` in `heroku` NPM script

[domoscargin]

What

Create a heroku.js script which handles at least the gulp copy-assets && gulp sassdoc portion of the heroku script in package.json.

Why

We have made the decision to move away from Gulp and write our own build scripts.

The heroku NPM script calls tasks through Gulp, which breaks when these tasks are updated to remove Gulp (eg: #2767).

We should ensure our NPM scripts have no gulp dependencies.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• [ ] Gulp dependency in the heroku NPM script has been abstracted into a separate JS file, or removed entirely

[domoscargin]

Moving this back to the backlog, as its dependent on updating the compilation tasks.

[domoscargin]

Going to close this in favour of https://github.com/alphagov/govuk-frontend/issues/2717

Think we need a discussion on the current state of this to prioritise future work.