new icon: css (original, plain)

[twonfi]

Double check these details before you open a PR

• PR does not match another non-stale PR currently opened
• PR name matches the format new icon: Icon name (versions separated by comma). More details here
• PR's base is the develop branch.
• Your icons are inside a folder as seen here
• SVG matches the standards laid out here
• A new object is added in the devicon.json file at the correct alphabetic position as seen here

This PR closes #2303

Link to prove your SVG is correct and up-to-date.

CSS-Next/logo.css

Note: The SVG files are resized to 128x128 and optimized. The plain SVG (with the transparent "CSS") was derived from the original SVG (with the "CSS" in white) plus some work done by me to merge the two paths into one.

Also, I used css instead of css-official-logo due to naming conventions (css better describes the CSS technology).