Add support for import at-rule layer functionality

[puckowski]

What:

Add support for @import layer functionality documented in https://developer.mozilla.org/en-US/docs/Web/CSS/@import

Builds on top of #4337

@import url("theme.css") layer(theme);
@import url("features.css") layer(features) supports(display: grid);
@import url("responsive.css") layer(responsive) supports(display: flex) screen and (max-width: 768px);
@import url("print.css") layer(print) print;

The above syntax has widely available browser support according to MDN.

Why:

Supporting the additional @import CSS syntax may benefit Less users.

Checklist:

• Documentation
• Added/updated unit tests
• Code complete

I did not venture into .less imports using this syntax as existing import code makes a clear distinction between .css and .less and the two have significantly different behavior.