Allow leading pipes for type aliases

[jeffmo]

Ocaml allows this on pattern matches and it's convenient for larger unions that span one-per-line.

This adds support for this to the flow parser:

type Nodes = 
  | {type: "Identifier", ... }
  | {type: "Expression", ... }
  | {type: "Literal", ... }
  ...
;

Note that this adds leading-| support for type aliases only for now. We could easily expand to other annotations, but I figured it's most useful for type aliases of large unions -- so might as well start there and leave other things constrained for now. Should be easy to open this up to all annotations in the future if we have a good reason to.