Add support for modern colour formats

[unikitty37]

When I highlight rgba(10 10 10 / 10%), right-click, and choose "Color to HSL", I get an error saying "Can't resolve color format of rgba(10 10 10 / 10%)".

I have to change it to the legacy format rgba(10, 10, 10, 0.1), at which point I can convert it. It generates hsla(0, 0%, 4%, 0.1), which is also the legacy format.

But this generates a Stylelint violation "Expected 0.1 to be 10% (alpha-value-notation)". If I make that change, I get "Expected modern color-function notation (color-function-notation)".

Stylelint is expecting the modern format: hsla(0deg 0% 4% / 10%); examples are at the link above. (It appears that the deg is required by a separate rule: hue-degree-notation.)

Would it be possible for the extension to:

1. read the modern formats, and
2. have an option to write colours in the modern formats rather than the legacy ones? (It would probably be a good idea to have a second option to control whether to use deg in angles, too.)