add types declaration file for TS support

[andersonjoseph]

closes #14

[kachkaev]

Thanks for this PR @andersonjoseph! @btd WDYT?

[kachkaev]

In the meantime, I am using declarations/sharp-phash.d.ts as a workaround:

declare module "sharp-phash" {
  import type sharp from "sharp";

  type SharpImage = Parameters<typeof sharp>[0];
  type SharpOptions = Parameters<typeof sharp>[1];

  export default function phash(
    image: SharpImage,
    options?: SharpOptions,
  ): Promise<string>;
}

declare module "sharp-phash/distance" {
  export default function distance(a: string, b: string): number;
}

Thanks for crafting the diff @andersonjoseph 🙌