feat(back-button): add variables and support for focused and hover states (#18451)

- updates MD spacing for back button text
- also adds e2e tests and adds them to screenshot

references #18279 references #17624 fixes #18465

core/src/components/button/button.scss

@@ -238,11 +238,6 @@
   border: 0;
 }
 
-
-// .button > .button-container:hover {
-//   opacity: var(--opacity-hover);
-// }
-
 .button-inner {
   display: flex;