Add SVGZ format

[abhro]

SVGZ is a layer of gzip compression over .svg files, and help with storage of plain-text vector graphics files. I'm not sure what the mimetype is, but support for .svgz can be incredibly helpful for saving full-quality graphics at smaller sizes.

• Spec: https://www.w3.org/TR/SVG/single-page.html#conform-ConformingSVGServers
• PR using it: None yet, trying to figure out how to get Makie to support saving to svgz