Add additional template directories to search when loading email templates.
The hook should return an array of directory strings. Elements at the end of the array are search first. Elements at the start of the array will be searched last.
The default locations searched are (in order):
array $default_templates Array of templates PMPro checks by default.
string $page_name The specific template that PMPro is looking for.
string $type The type of template, e.g. “pages” or “email”.
string $where Defaults to “local” to search local directory paths, but could be “url” to search for templates using URL paths.
string $ext The extension to add to the template name, e.g. “.php” or “.html”.