fix(label): move default flex and margin to item (#16461)

This moves item-specific margin and flex values out of label and into item as a slotted style to prevent those styles from leaking into labels further down the tree.

closes #15393

core/src/components/label/label.scss

@@ -10,12 +10,8 @@
    */
   --color: initial;
 
-  @include margin(0);
-
   display: block;
 
-  flex: 1;
-
   color: var(--color);
 
   font-family: $font-family-base;