feat(pepicons.com): Favourite icons

### UI
- heart icon to left of icon modal
- same color as bottom section
- same size as the code buttons
- should always use `type="pencil"`
- "favourited" should show `heart-filled` instead, and have the "main" color that the user picked


### Behaviour
- when you click to favourite an icon, it should save it to localStorage
- IF the local storage is empty and no icons have been saved yet
- the modal should close and the website should smooth scroll to the top
- there as a first _new_ category we see `Favourites` and there the icon you just favourited is seen
- IF we already have 1 or more icons favourited, then we just make the heart icon the main color without closing the modal

be sure the favourited icons are persisted on page refresh