fix: missing type and altText from Image interface

[jclaessens97]

Comparing the output of an image of type Image to the interface Image, type and altText are missing:

output:

image: {
    type: 'image',
    read: [Function: read],
    readAsArrayBuffer: [Function: readAsArrayBuffer],
    readAsBase64String: [Function: readAsBase64String],
    readAsBuffer: [Function: readAsBuffer],
    altText: '6,297,900+ Photo Image Art Stock Photos, Pictures & Royalty-Free Images -  iStock',
    contentType: 'image/jpeg'
  }

I've added those 2 types to the index.d.ts