[css-fonts-4] [varfont] Allow font-variation-settings to use named values from STAT table

[robmck-ms]

The font-variation-settings property enables web designers to directly address instances of a variable font by numeric value. E.g. "font-variation-settings: 'FNCY' 9000". The difficulty is: for nonstandard axes (e.g. those created by the font designer), there's no way to know what numeric value to use for a given use. Font designers would have to post instructions with the font, and designers will need to remember or look them up.

The STAT table in an OpenType variable font gives font designers a way to name points along an axis. It would be nice if user-agents could use the information from that table to allow named values for axes. E.g. "font-variation-settings: 'FNCY' 'Super Fancy'" or "font-variation-settings: 'FNCY' 'boring'".

[svgeesus]

Related: #525

[Crissov]

and #518

[litherum]

font-variation-settings isn't the best place for this, given its intended use is just a backdoor to get access to axes that are otherwise impossible to access. Indeed, font-variation-settings should almost never be used. Instead, we are discussing more general solutions for named values in #525. Let's move this specific discussion to there.