Adding blueprint.json for enabling plugin previews in WP.org plugin repo

[ashfame]

This PR moves the assets directory one level under .wporg so that blueprint generating script and setup php code can reside there.

The idea of the helper files (blueprint_runphp_step_code.php and update_blueprint.sh) is to easily manage the setup PHP code in an actual PHP file and using the bash script to update the runPHP step's code in blueprint.json.

Playground preview based on the blueprint file in this PR - https://playground.wordpress.net/?blueprint-url=https://raw.githubusercontent.com/automattic/chatrix/blueprint/.wporg/assets/blueprints/blueprint.json