Add explicit return type declarations to PHP methods for WP Framework compatibility

[jamesmorrison]

Requires wp-framework 2.0.0 / develop

Description of the Change

Add explicit return type declarations to PHP methods for WP Framework compatibility
Adds strict types to all PHP classes

Closes #

How to test the Change

1. Create a site using the WP Scaffold
2. Run composer install; npm install; npm run build for:

• The MU plugin: mu-plugins/10up-plugin
• The 10up theme: themes/10up-theme
• The 10up block theme: themes/10up-block-theme

3. Confirm there is no fatal error in WP Admin
4. Activate the 10up theme; confirm no fatal errors
5. Activate the 10up block theme; confirm no fatal errors

Changelog Entry

Added: explicit return type declarations to PHP methods for WP Framework compatibility

Credits

Props @jamesmorrison

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added Critical Flows, Test Cases, and/or End-to-End Tests to cover my change.
• All new and existing tests pass.