Add support for lists

[Martin Janiczek]

This means:

• add List (List Expr) constructor Frontend.Expr type... solve any compiler errors that arise as a result (emitExpr etc)
• add tests that show all the various ways lists can be written work. For example:
  • [], [ ]
  • [1], [ 1 ]
  • [1,2], [ 1, 2 ]
  • I can't think of any more. Let's not deal with newlines right now as that will be more tricky and done in another issue.
• partially implement type inference - focus on the items: all items inside list have to have the same type. Should be enough to eg. for list [A,B,C] do equals typeA typeB, equals typeA typeC. Don't have to do all the combinations. This will leave us to do the type of the actual list itself in a later issue after we implement type parameters.
• add emit tests too:
  • Elm's [ 1, 2, 3 ] ➡️ JavaScript's [1, 2, 3]
  • [] ➡️ []
• implement emitting to pass the emit tests above
• update the "what's done" table in README for lists: parser tests, desugaring (none), type inference (only partially = ⚠️), emit and emit tests... but not parsers and optimizations.

If you get stuck on anything, ask here or on Discord! We'll help you 🙂

Out of scope ❌ ❌ ❌

• tests for newlines support in lists
• parser for lists
• optimizations (they'll become available with the operators but not sooner I think ... at least I can't think of any)
• type inference of list itself
• (::) operator
• (++) operator