index.d.ts still allows callback based handlers.

[Caleb Everett]

Describe the bug
The third argument of MiddyInputHandler is documented as callback: () => ..., but in practice it is { signal: AbortSignal }`.

To Reproduce

    const wrapped = middy((e, c, callback) => {
      callback satisfies () => void
      expect(callback).toEqual({
        signal: expect.any(AbortSignal),
      })
      callback(null, {});
    })

    return new Promise((resolve, reject) => {
      wrapped({}, {}, (err, res) => (err != null) ? reject(err) : resolve(res));
    })

Expected behaviour

• Callback handlers should be a type error. Middy never calls the callback argument of a middified handler, and it never calls a wrapped handler with a callback argument.
• The signal argument should be documented in the index.d.ts file.