Add a lint for unnecessary @at-root

[nex3]

I've seen a number of cases where authors will write styles like @at-root .foo & or even @keyframes { @at-root from { ... } }, where the @at-root rule is totally unnecessary (in the first case because the selector includes & outside of any interpolation, so Sass already knows not to implicitly add a leading &; in the second case because @keyframes is always bubbled up to the root anyway). It would be useful to have a lint that advises against doing this and suggests deleting the @at-root as a fix. Ideally it would work with both the inline selector format and the nested format.