Added grunt webpack:watch task

[abotteram]

Context

• Create an easier way to work with javascript without YOAST_SEO_DEV_SERVER.

Summary

This PR can be summarized in the following changelog entry:

• Adds a grunt webpack:watch task that quickly rebuilds javascript when changing a js file without having to run grunt build:js again.

Relevant technical choices:

Test instructions

This PR can be tested by following these steps:

• Run grunt webpack:watch.
• Make a change to a JS file which you can easily see (like adding a console.log to admin-global.js), and save it.
• See that grunt webpack:watch almost instantly rebuilds the javascript.
• Reload the browser to see the change you made.

UI changes

• This PR changes the UI in the plugin. I have added the 'UI change' label to this PR.

Documentation

• I have written documentation for this change.

Quality assurance

• I have tested this code to the best of my abilities
• I have added unittests to verify the code works as intended

Fixes #

[hansjovis]

CR: 👍

[hansjovis]

Acceptance test: ✅

Indeed a lot faster than grunt build-watch 🎉 .