DuplicateProperty should not produce a lint when including a fallback for env()

[srawlins]

iOS 11 and the iPhone X have the notch thing, and safe areas and all, which can be styled around using env() (or constant() in iOS 11.0 I guess). But fallbacks still need to be produced for browsers that don't understand env(). Taken from an ayogo blog post:

header {
    /* ... */

    /* Status bar height on iOS 10 */
    padding-top: 20px;

    /* Status bar height on iOS 11.0 */
    padding-top: constant(safe-area-inset-top);

    /* Status bar height on iOS 11+ */
    padding-top: env(safe-area-inset-top);
}

DuplicateProperty should recognize that: properties with a value that is an env() function call (or I suppose constant() function call) should not be considered duplicate.

In the meantime of course, you can use ignore_consecutive.