Update icons

[planktonic]

• When the new icon set is finalised, we should update the Icon component used here in our docs. The new icons should either be provided here, or imported from an external package.
• Icons used in XSmall Square & Round buttons should be the 20dp icons.
• Tab button, Icon only has 16px padding, should be 12px. Can we do that somehow, avoiding making button styles dependent on content, and without introducing a new button type. Perhaps just the square & round buttons could have their padding set to zero, so the min-height and min-width properties can be solely responsible for the button size
• Update badge-button icons to use "X" icon in examples
• atoms/flash: use Outline icons not filled
• our buttons currently output icons with a-icon--m. Let’s switch to not outputting a size unless needed (e.g. perhaps small buttons should get small icons)