import/export game state

My tentative idea is add a savegame system where you have 3 or more slots (each their own unique game state) that you can manage in a simple UI and you can paste in a string from your clipboard or export an existing save to your clipboard. Minified JSON seems like the most straighforward. Not gonna overthink the schema too much as we can just detect a schema version and parse it appropriately. That is, if we need to update the schema we'll be able to easily do so, and people will copy either v1 or v2 codes, for example.