Improve critical path + clean unused css

[chicio]

Remove critical path from styles and remove unused rules from them using an npm library.

Description

New gulp task that uses a library to remove unused rules. Upgrade of critical in order to be able to use the callback with css + uncritical to save them.

Motivation and Context

Speed 🏃

How Has This Been Tested?

localhost tests.

Types of changes

• Bug fix 🐛 (non-breaking change which fixes an issue)
• New feature ✨ (non-breaking change which adds functionality)
• Breaking change 💥 (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project 🍻.
• My change requires a change to the documentation 💡 and I have updated the documentation accordingly.
• I have read the CONTRIBUTING document 👥.
• I have added tests to cover my changes 🎉.
• All new and existing tests passed ✅.