feat(server): useProps context

[deer]

closes #755

This change allows things like this:

export default function Grandchild() {
  const props = useProps();
  if (props.data === "computing this is really hard") {
    return <div>{props.url.toString()}</div>;
  }
  return <div></div>;
}

So components can get access to the props from the root component by using the newly introduced useProps context.

This is a generalization of the PR that was originally opened to close this issue. I thought it might be convenient to have not just the route available, but the entire props. I'm opening this as a draft since I didn't write any documentation yet, and I'm concerned about performance implications.

@marvinhagemeister, would you be so kind to clarify what sort of performance problems this will introduce? I see https://stackoverflow.com/questions/75060633/react-context-performance-and-suggestions and they talk about when context changes, the components re-render. Since we're just rendering this all on the server and the context is only created once and never changes, maybe it doesn't matter.

Mildly related to #1339.

[marvinhagemeister]

@deer Performance of context providers is not a concern on the server. It's mostly just a property access which costs nothing. It matters much more on the client where things are stateful and context updates can cause an app to re-render.

[deer]

Thanks for the clarification Marvin. I've tried my hand at writing some documentation for this and have therefore marked it as ready.

[deer]

Another FYI: we'll need to update the ToC to include this new example when this feature gets released in the next version. Currently the doc is just in the canary ToC. (Assuming this gets approved!)