Add CSS variable fonts syntax to the main spelling library

Problem/Motivation

Context: https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_fonts/Variable_font...

font-variation-settings settings are prohibited by dictionary check.
https://developer.mozilla.org/en-US/docs/Web/CSS/font-variation-settings

But these are quite useful properties. So we should add them.

I added registered properties
https://developer.mozilla.org/en-US/docs/Web/CSS/font-variation-settings...