Add `.d.ts` TS ambient

[hackape]

It looks like this lib is practically in archive mode, so I figure I'll just post this ambient declaration here in a issue instead of a PR. Whoever wants to scratch the same itch like me, copy the following and paste into src/global.d.ts (filename actually doesn't matter, as long as it ends in .d.ts) in your project's directory.

declare module "mnist" {
  type Output = [number, number, number, number, number, number, number, number, number, number];

  type Datum = {
    input: number[];
    output: Output;
  };

  interface Digit {
    id: number;
    raw: number[];
    length: number;
    get: (index?: number) => number[];
    range: (start: number, end: number) => number[][];
    set: (start: number, end: number) => Datum[];
  }

  namespace MNIST {
    export function set(
      trainingSetSize: number,
      testSetSize: number
    ): {
      training: Datum[];
      test: Datum[];
    };

    export function get(count: number): Datum[];

    export function draw(
      digit: number[],
      context: CanvasRenderingContext2D,
      offsetX: number,
      offsetY: number
    ): void;

    export function toNumber(array: number[]): number;
  }

  export = MNIST as typeof MNIST & Digit[];
}