feat(ssr): render portals

[dsseng]

It's probably not the best possible solution, so I'm waiting for comments for improvements.

[yyx990803]

I think it'd be more flexible to get portal content like this:

const portals = {}
const html = await renderToString(app, portals)
console.log(portals['#modal'])

[dsseng]

@yyx990803 should they be then received by server rendering code and moved where required, right? I can do that.

[yyx990803]

I was testing this locally and realized it's better to pass the ssr context via the app context instead. Handles concurrency and doesn't require nested passing.

The previous passing implementation would also require the compiler to generate different code (in order for renderComponent calls used in ssr render functions to use the correct context)