[Button] Add “pressed” state

[dleroux]

Problem

Sometimes buttons are used like a radio button, usually in a group but potentially on their own (as a toggle).

As implemented in polaris-rails: https://github.com/Shopify/polaris-ux/issues/58

Examples

[dleroux]

@maxariss

[yourpalsonja]

@dleroux can we close this in favour of the ToggleButton component that is currently in polaris-next?