Pipe (->) autocompletion for built-in types.

[Cristiano Calcagno]

• array<> could by default autocomplete from Js.Array2.t<>
• notice Belt.Array.t<> is not defined, an oversight -- can be added.
• list<> there's no module Js.List2 at the moment, which seems inconsistent.
• string not sure but see #99
• option<>

[Sergey Samokhov]

there's no module Js.List2 at the moment, which seems inconsistent.

But Js namespace is for zero-cost bindings to functions/methods built into JS engines, and there are no built-in linked lists in JS.