Update blueprint admin-notice to use the new writeFiles

[bph]

Current step:

{  "step": "writeFile",
            "path": "/wordpress/wp-content/mu-plugins/bgnightly-notice.php",
            "data": "<?php add_action('admin_notices', function() {  echo '<div class=\"notice notice-info is-dismissible\"><p>Hello from Playground.</p></div>'; });"
        }

Becomes:

{
  "step": "writeFiles",
  "writeToPath": "/wordpress/wp-content/mu-plugins",
  "filesTree": {
    "name": "mu-plugins",
    "files": {
      "bgnightly-notice.php": "<?php add_action('admin_notices', function() {  echo '<div class=\"notice notice-info is-dismissible\"><p>Hello from Playground.</p></div>'; });"
    }
  }
}

The key differences are:

• uses writeFiles instead of writeFile
• specifies the parent directory in writeToPath
• creates a file tree structure with the file to write
• The file content remains the same

This achieves the same result - creating a must-use plugin that displays a notice in the WordPress admin, but uses the new directory-based approach.

[bph]

there is actually nothing to gain by using writeFiles as this blueprint only Create a single file in the mu-plugins folder.