feat(@clayui/css): Mixins clay-css adds option to output a Sass `@e…

…xtend` with the `extend` key

How to use:
```
$btn-palette: (
    '%btn-custom': (
        color: blue,
    ),
    '.btn-custom': (
        extend: '%btn-custom',
    ),
);
```

packages/clay-css/src/scss/mixins/_globals.scss

@@ -274,6 +274,8 @@
 							transition: none;
 						}
 					}
+				} @else if ($key == 'extend') {
+					@extend #{$value} !optional;
 				} @else if ($key == 'user-select') {
 					-ms-user-select: $value;
 					-moz-user-select: $value;