Architecture Update/Migration

### Description

* Migrate the contents of the `/php/` folder to the `/src/` folder and apply best practices from [create-wordpress-plugin](https://github.com/alleyinteractive/create-wordpress-plugin).
* Organize code into [features](https://github.com/alleyinteractive/wp-type-extensions/tree/main/src/features) where applicable.
* Remove the legacy code from wp-seo.php.
* Features should be grouped
* This will require critical thinking regarding what constitutes a feature and which features should be dependent on one another
* Suggested baseline features should be recommended.
* For v2.0.0 features should be maintained in code, but should be surfaced in the Admin-side as readonly for v2.0.0
* Features should be opt-in
* v2.0.0 can be composer-install only (WP bundles can come later)

### Use Case

Continue rebase of plugin on create-wordpress-plugin. Plugin should be feature-based.