Render builtin types with type variables

[hojberg]

There's a very specific set of builtin types in Unison and we can render them more nicely than builtin type.
For instance, base.List could be rendered as type List a (or perhaps builtin type List a). Showing type variables helps communicate how the builtins are constructed.

The full list of builtins are here: https://github.com/unisonweb/unison/blob/trunk/parser-typechecker/src/Unison/Builtin.hs#L153

Of those, these are the ones with type variables:

• type MVar a
• type TVar a
• type Ref {g} a
• ability Scope s
• type Request {g} r
• type List a

We can hard code this rendering in the UI for now.
Note that builtins have special hashes: like ##Nat for instance.

[hojberg]

After working with this for a bit, I'm starting to reconsider that this should exist in the UI.
Essentially the syntax being generated are like type "summaries" (see unisonweb/unison#2175), and implementing this as such in the backend seems like a better approach.