Print optional arguments in TypeScript as foo?: type

[alexcrichton]

Right now

foo: func(a: option<u32>)

generates:

export interface Input {
  foo(a: number | null): void;
}

where it should ideally instead generate:

export interface Input {
  foo(a?: number): void;
}

[lucasavila00]

I think the proposed approach will not work due to following - or it should fall back to unions in such cases

// foo: func(a: option<u32>, b: u32)

export interface Input {
  foo(a?: number, b: number): void;
  //              ^ A required parameter cannot follow an optional parameter.ts(1016)
}