feat: Add strict types and TSDocs to ImageScript.d.ts (#43)

This PR adds detailed type declarations and TSDoc comments for all exported members, as well as several new `type` and `interface` exports for ease of use.

E.g.:

```ts
export class Image {
  // ...

  /**
   * Encodes the image into a JPEG.
   *
   * @param quality `1`-`100`, where `1` is lowest quality (highest compression)
   *     and `100` is highest quality (lowest compression). Default: `90`
   */
  async encodeJPEG(quality?: JPEGQuality): Promise<Uint8Array>;

  // ...
}

/**
 * - `0` = **lowest** quality (smallest file size)
 * - `100` = **highest** quality (largest file size)
 */
export type JPEGQuality =
  | 1
  | 2
  | 3
  // ...
  | 98
  | 99
  | 100;
```

Author: acdvorak