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-close.ts

@@ -13,14 +13,14 @@ import { MenuController } from './menu-controller';
  * A simple `menuClose` button can be added using the following markup:
  *
  * ```html
- * <button menuClose>Close Menu</button>
+ * <button ion-button menuClose>Close Menu</button>
  * ```
  *
  * To close a certain menu by its id or side, give the `menuClose`
  * directive a value.
  *
  * ```html
- * <button menuClose="left">Close Left Menu</button>
+ * <button ion-button menuClose="left">Close Left Menu</button>
  * ```
  *
  * @demo /docs/v2/demos/menu/