Added file style.css which is aimed to contain CSS code that affects the look of the gallery. This could be extended to provide different or user-modified styles to give each gallery an individual look.
JavaScript files are now in js/, CSS files in css/, thumbnail images in thumbnails/.
This export module should replace the PhotoSwipe copy in darktable. See https://github.com/darktable-org/darktable/issues/16205 for more details.