Implement unit value and type

[Martin Janiczek]

() should be a valid Elm syntax. What we emit it as is probably not that important right now (we will find out later!) - let's emit as {"type": "unit"}

That means we need to implement:

• parser tests
• emit tests (if done after #7 )
• new AST.Frontend.Expr case
• parser
• new type (treat it similarly to literals in type inference)
• JS emit case

No optimizations needed (or at least I can't think of any), no desugaring needed either.

The compiler errors will probably guide you 😅 Ask here or on Discord if anything is unclear!