feature: group import statements

I mentioned this in another issue but never made it a request of its own. The request is to group import statements together:

@import (reference) {
    "one.less",
    "two.less",
    "three.less",
    "four.less"
}

I suppose we would have to adjust syntax for media queries, maybe something like:

@import (rule) (min-width:400px) {
    "one.less",
    "two.less",
    "three.less",
    "four.less"
}

Another option would be to just group the files together like this:

@import (rules) "one.less, two.less, three.less" (min-width:400px);

But this doesn't seem like a good syntax for a bunch of files like you might have with a lib like Bootstrap.

Start by reviewing issue #1334 and its discussion of grouped @import syntax, including the proposed handling for media queries. No files, tests, or entry points are named; done would require an agreed syntax and corresponding compiler behavior and tests.