replace the DIRECTORY_SEPARATE with slash for other OS

[vodanh1213]

This PR will ensure that if the OS is not *nix, then the DIRECTORY_SEPARATOR in the $path will be parsed correctly, along with the $plugin_dir.
Problem:

public static function set_path( string $path ) {
    $plugin_dir = WP_PLUGIN_DIR;
    if ( DIRECTORY_SEPARATOR !== '/' ) {
        $plugin_dir = str_replace( DIRECTORY_SEPARATOR, '/', $plugin_dir );
    }
    $plugins_content_dir_position = strpos( $path, $plugin_dir );
    $themes_content_dir_position  = strpos( $path, get_theme_root() );
    ...
}

The $plugin_dir gets parsed, while the $path is still contains the different DS, which will make the check $plugins_content_dir_position = strpos( $path, $plugin_dir ); always false, such as C:\sites... vs C:/sites/...