fix(extractImages): calculate channels correctly and return image metadata

[Yiin]

Calculate image channels based on data length, width, and height.

extractImages now returns an array of objects containing data, width, height, channels, and key instead of just the raw data array. This fixes issues with distorted images caused by incorrect channel assumptions and provides necessary metadata for downstream processing.

resolves #21

Example of extracting images to png format:

const imagesData = await extractImages(pdf, pageNum);

for (const imgData of imagesData) {
  const imageIndex = totalImagesProcessed + 1;

  try {
    await sharp(imgData.data, {
      raw: { width: imgData.width, height: imgData.height, channels: imgData.channels }
    })
    .png()
    .toFile(`${imageIndex}.png`);
  } catch (e) {
     // handle error
     continue;
  }
}

It is a breaking change because return value was changed, but given that this function was unusable before I have doubts it was in use anywhere

[johannschopplich]

Thanks a lot!