[WIP][rtl][css] Fix button margins for RTL langs

When showing a button in RTL, margin-left/right shows up
incorrectly. The needed CSS seems to be `margin-inline-start/end`.

Those values do not exist in IE, so not sure if we can introduce these or not just yet.

src/global_styling/mixins/_button.scss

@@ -72,16 +72,16 @@
     flex-direction: row-reverse;
 
     > * + * {
-      margin-left: 0; // 1, 2
-      margin-right: $euiSizeS; // 1, 2
+      margin-inline-start: 0; // 1, 2
+      margin-inline-end: $euiSizeS; // 1, 2
     }
   } @else {
     display: flex;
     justify-content: center;
     align-items: center;
 
     > * + * {
-      margin-left: $euiSizeS; // 1
+      margin-inline-start: $euiSizeS; // 1
     }
   }
 }