Feature/add modular plugin scaffold

[dsawardekar]

Requirements

Modular Plugin architecture for 10up wp-scaffold

Description of the Change

This PR add an mu-plugin for a modular object-oriented Plugin architecture including,

• Post Type Builder
• Taxonomy Builder
• Assets Manager
• Site-wide Settings
• PHPUnit Test Suite

Alternate Designs

NA

Verification Process

Run vendor/bin/phpunit to verify that the test suite passes.

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.