Sapper: Support for preload function

[dummdidumm]

Is your feature request related to a problem? Please describe.
Using sapper with the preload function inside module script gives a bad DX.

export async function preload(page, session) { // <------- not typed, need to add typings yourself
    const res = await this.fetch(`blog/${slug}.json`); // <--- error, what it `this`?
}

Describe the solution you'd like
Support typing for the preload function and for this, but in such a way that it will not also provide these for non-sapper-projects.

Notes / open questions

• this probably needs to be transformed to some ambient global which has fetch defined on it.
• preload is tricky because it's not a usage of something we can provide in the ambient context, but a definition . Not sure how to implement this.
• How to only provide these typings/transformations in a Sapper project? How can we determine this? Look into the package.json?

[dummdidumm]

After some more thought and hearing that the context is possibly user-modifiable in the future, I'm deciding against baking this into the language-tools.

• getting arguments ambiently typed is not possible
• typing this is possible to do by the user, and since it may be modified, our typings may be wrong anyway in the future

How to type this this (hah) then?

You can type the this context of a function explicitly in TypeScript (see docs):

     // Better move that into some file and do `import type { PreloadContext } from ...` to not type it every time
    interface PreloadContext {
        fetch: ...,
        ...
    }

    // won't work with export const preload = (...
    export async function preload(this: PreloadContext, ...page/session, if you need it) {
       ...
       this.fetch(...) // <-- works without type errors
    }

[dummdidumm]

Fixed via sveltejs/sapper#1468