Test image generation & markup with non-photographic images

[rootwork]

We already have a note that it's maybe not ideal, but we should test it to see how it responds if you send it a GIF or an SVG.

[rootwork]

Note has been expanded and modernImages option has been added so that it can be quickly disabled. Remaining to-do list:

• Provide image minification even when the option is not enabled, otherwise it just copies the existing file.
• Align and possibly move imageFormats with/to the screen sizes in tailwind.config.js.
• Provide a way to handle SVG images, which are not currently included as they are not able to be processed by Sharp.

Based on Sharp and gulp-sharp-responsive docs, the image types it can process are jp(e)g, png, webp, gif, avif, heif, tiff -- but in testing it choked on sample avif and heif files, so we won't enable those by default. It would not handle older image formats like bmp, uncommon formats like jfif, or non-image formats like pdf, but with the recent code changes we're only feeding it the files it can handle, rather than throwing the whole directory at it.