feat(@clayui/css): Mixins clay-css should output any CSS Custom Properties passed into it

[pat270]

fixes #4252

This allows passing CSS Custom Properties to any Sass map that uses the clay-css mixin.

$btn-primary: (
    --btn-primary-background-color: blue,
);

.btn-primary {
    @include clay-css($btn-primary);
}

should output:

.btn-primary {
    --btn-primary-background-color: blue;
}

[matuzalemsteles]

LGTM! That's cool, thanks @pat270