[css-fonts] Access arbitrary points on a variable 'ital' axis via `font-style`

The Microsoft definition of the ital variable font axis (https://docs.microsoft.com/en-us/typography/opentype/spec/dvaraxistag_ital) says,

Valid numeric range: Values must be in the range 0 to 1.

Scale interpretation: A value of 0 can be interpreted as “Roman” (non-italic); a value of 1 can be interpreted as (fully) italic.

...

The Italic axis can be used as a variation axis within a variable font, though this is not expected to be common.

The Italic axis is distinct from the Slant axis ('slnt'). Fonts may use one or the other, depending on the nature of the design, but should rarely use both.

While "not expected to be common" and "should rarely use both" are niceties, there are now such fonts in the wild (eg https://djr.com/roslindale/ - see https://v-fonts.com/fonts/roslindale-variable-italic)

Therefore the current definition of https://www.w3.org/TR/css-fonts-4/#font-style-prop has a problem, because font-style cannot access arbitrary points on a variable 'ital' axis.

Start with the CSS Fonts Level 4 definition of the font-style property and compare its handling of the ital axis with the linked OpenType definition. Review the Roslindale example to understand the use case; done means the specification explains how font-style can access arbitrary ital-axis values.