feat: Add APIs for watermarking images

[mrkurt]

This adds image.overlayWith and image.extend, both useful for watermarking images.

Here's an example, it takes a picture of a hot air ballon, and puts the Fly logo on the bottom right corner (with a little bit of padding):

    const img = new Image(picture)
    const watermark = new Image(overlay)
    const meta = img.metadata()
    const wmeta = watermark.metadata()

    img.overlayWith(watermark, { gravity: Image.gravity.southeast })
    const img2 = await img.toBuffer()