Add option to use Merge from type-fest to better conversion of object type spreads

[kevinbarabash]

Object type spreads in flow result in properties that come later overriding those that come sooner, e.g.

type A = {foo: string};
type B = {foo: number};
type C = {...A, ...B} // same as {foo: number}

In TypeScript if we do type C = A & B we end up with {foo: never} because the intersection between string and number is empty.

type-fest has a bunch of utility types including Merge which provides the same functionality as Flow's type spreads.

Does anyone has code the relies on Flow's object type spread functionality?