SSR guide: add directive transforms

[jan-thoma]

The Documentation for writing SSR Transforms for directives, beside a very basic noop example, is nonexistent. I've written an Image Lazy Loading Directive and want to provide an SSR Transform. adding simple properties like src or poster are working flawless. When it comes to adding a style attribute the lack of documentation makes it very hard find a solution. The transform isn't very complex, but i'm struggling to add style='background-image:url()". See the Transform code below

export const vueLazyLoaderSSRTransform = (dir, node, context) => {
    if (node.tag === 'img') {
        return {
            props: [createObjectProperty('src', dir.exp)]
        }
    }

    if (node.tag === 'video') {
        return {
            props: [createObjectProperty('poster', dir.exp)]
        }
    }

    return {
        props: [
            createObjectProperty('style',
                createObjectExpression([
                    createObjectProperty(
                        'background-image',
                        createSimpleExpression('url()', true)
                    )
                ])
            )
        ]
    }
}

It would be great to have a more in depth documentation on SSR Transforms

[skirtles-code]

I think this is the same issue as #1122. Not sure why that was closed, but we can use this one instead.