implement config.host and config.hostHeader

[Rich-Harris]

ref #207. This implements a new config.host option which overrides req.headers.host on the server and location.host in the client. This is particularly useful in the context of prerendering, where there's no such thing as req.headers.host because there's no req.

TODO: implement config.hostHeader, allowing host to be controlled by e.g. req.headers['x-forwarded-host']

[benmccann]

I think most of the adapters will need to be updated as well?

kit/packages/adapter-vercel/src/index.js

Line 10 in c3e7719

host: null, // TODO

[Rich-Harris]

Maybe not, actually — I think it can all be done inside the renderer itself, meaning it doesn't need to be part of the request object passed to the renderer

[benmccann]

we'll have to at least remove the TODOs in that case 😄