Support Context API in Declarative Shadow Dom slotted elements (web-components SSR)

[aralroca]

Related with #19

It's the missing piece to make the Context API work well:

Done before:

• (server): Server Components
• (client): Web Components after hydration, supporting slots
• (server): SSR Web Components SSR, like Server Components that doesn't have slots

To do in this task:

• (server): Support slots during SSR Web Components

To do that we are using the slot behavior of Declarative Shadow Dom (way to support SSR of web-components). We will have to change the renderToReadableStream logic to support slots.