Distinction between nullable and optional

[GerritPlehn]

Hi! Similar to #21 when using this library to generate TS types, it's often useful to make a distinction between nullable and optional fields.
I'm looking for feedback on this PR as I'm very new to Rust. Currently not all tests are passing and before investing more time I'd like to know if the approach I took is ok.

Input

{
  "a": [{ "nullable": 1 }, { "nullable": null }],
  "b": [
    {
      "optional": 1,
      "always": true
    },
    {
      "always": true
    }
  ],
  "c": [
    {
      "optionalNullable": 1,
      "always": true
    },
    {
      "optionalNullable": null,
      "always": true
    },
    {
      "always": true
    }
  ]
}

Before the changes

export interface A {
    nullable?: number;
}

export interface B {
    optional?: number;
    always: boolean;
}

export interface C {
    optionalNullable?: number;
    always: boolean;
}

After the changes

export interface A {
    nullable: number | null;
}

export interface B {
    optional?: number;
    always: boolean;
}

export interface C {
    optionalNullable?: number | null;
    always: boolean;
}

[evestera]

Thank you for the PR. Looks reasonable at first glance, but it's been a while since I worked on this project, so will need to re-familiarize myself with the project a bit before I consider merging.