Add inventory

[CarolineClark]

The spellbook works for a small number of commands, but cannot display more in depth information (e.g. how do we show information for ls -a?)
We could instead move the information to an inventory (that you open with Esc, or clicking a button)

We can then add extra information like:

• Weapons: "equipped" aliases
• Map/current location in world: cd location
• Appearance of user: they are asked to choose an outfit from the start, so they could be shown how they look in ascii
• More in depth information about the commands they've unlocked.
• Side quests available
• Stats