Option to customize links for serialization

[siliconeidolon]

I saw that the option to customise the key for link destinations was added to the deserialization function, but it'd be good to have it in the serializer as well.

I expect we'd need to add linkDestinationKey to the serializer options, then use it here:

case nodeTypes.link:
      return `[${children}](${(chunk as BlockType)[linkDestinationKey] || ''})`;

Unfortunately, BlockType expects 'link' only, so I'm not sure whether it would make sense to make that property type more permissive or maybe I'm missing an alternative way to handle this. Happy to submit a PR with some guidance.

Example Slate object:

{
    "type": "a",
    "url": "https://google.co.uk", // referenced by `linkDestinationKey`
    "children": [
        {
            "text": "Google"
        }
    ],
}

Would convert to

[Google](https://google.co.uk)