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/menu-button/menu-button.md.scss

@@ -32,7 +32,6 @@ ion-icon {
 // Menu Button Color: Focused
 // --------------------------------------------------
 
-
 :host(.ion-color.ion-focused) .button-native {
   background: #{current-color(base, .24)};
   color: #{current-color(base)};