Access to res.locals in Sapper store

[briancray]

I noticed you have access to a req object in the server side Sapper middleware, but I'd love to see access to res.locals, which is where we traditionally store stuff to be rendered to the view. We have several middleware that add information to res.locals as the request travels through our response pipeline.

[Rich-Harris]

Yeah, that sounds like a good idea, thanks. Would you be up for making a pull request? I think it'd just be a case of changing this line (and the associated TypeScript declarations) and augmenting/adapting this test.

[briancray]

This is great, thank you!