refactor(button): add ion-button attribute and icon attributes to style buttons

BREAKING CHANGES:

- `<button>` becomes `<button ion-button>`
- `<a button>` becomes `<a ion-button>`
- `<button ion-item>` does not get the `ion-button` attribute
- Buttons inside of `<ion-item-options>` do get the `ion-button`
attribute
- Removed the `category` attribute, this should be passed in
`ion-button` instead.
- Button attributes added for icons in buttons: `icon-only`,
`icon-left`, and `icon-right`

closes #7466

src/components/menu/menu.ts

@@ -125,7 +125,7 @@ import { GestureController } from '../../gestures/gesture-controller';
  * directive anywhere in the page's template:
  *
  * ```html
- * <button menuToggle>Toggle Menu</button>
+ * <button ion-button menuToggle>Toggle Menu</button>
  * ```
  *
  * To close a menu, add the `menuClose` button. It can be added anywhere
@@ -136,7 +136,7 @@ import { GestureController } from '../../gestures/gesture-controller';
  * <ion-menu [content]="mycontent">
  *   <ion-content>
  *     <ion-list>
- *       <button menuClose ion-item detail-none>Close Menu</button>
+ *       <button ion-button menuClose ion-item detail-none>Close Menu</button>
  *     </ion-list>
  *   </ion-content>
  * </ion-menu>