Add support for latest syntax in @import #20

For example :

@import url("green.css") layer supports(selector(&)) (min-width: calc(10px));

/* layer as a function */
@import url("green.css") layer(bar) supports(selector(&)) (min-width: calc(10px));

Existing things like 10px have correct syntax highlighting even in @import statement.

Missing syntax highlighting :

• declarations in supports()
• selectors in supports()
• any function

• specification : https://drafts.csswg.org/css-cascade-5/#at-import
• mdn docs : https://developer.mozilla.org/en-US/docs/Web/CSS/@import
• wpt : https://wpt.fyi/results/css/css-cascade?label=master&label=experimental&aligned&q=import